Guilds often hand out quests which can range from tutorial level such as household chores to much more difficult ones such as defeating hordes of fire breathing dragons There are many veteran adventurers who have completed numerous high level quests but very few are willing to offer their assistance to those starting out the journey as novice adventurers. The rewards from clearing the low rank quests are simply too cheap to worth the trouble and splitting it among the party makes it more miserable. The guilds are working hard to encourage more people to help out with the quests and improve on the high turnover rate due to poor job satisfaction and danger ahead.
The same way real guilds and colleges did/do
There's any number of ways to make this happen. I've taken all of these from real guild or academia practices.
- In order to become a master in the guild, a journeyman has to teach an apprentice until they pass their journeyman certification.
- To maintain mastery-level membership, a journeyman has to take on a certain number of apprentices, say, one every five years.
But regulations don't come into existence without reason. The real motivation for why a guild puts those regulations into place is that there are many good reasons to want an apprentice:
- It's great to have someone take care of your busy work so you can focus on your major quests. Fighting dragons is much easier if someone cleared out their goblin minions first. Copying over spells into your spellbook is a pain — it's nice to have someone else do it.
- Teaching is a valuable way to solidify skills and practice the basics.
- Constantly exposing yourself to younger students keeps you in touch with new developments in martial and magical practices.
- "The best swordsman in the world doesn't need to fear the second best swordsman in the world; no, the person for him to be afraid of is some ignorant antagonist who has never had a sword in his hand before; he doesn't do the thing he ought to." For a veteran adventurer, the average enemy is going to be much closer to their apprentice's level, which means that having an apprentice keeps them in practice fighting ignorant antagonists, which will serve them even better than practicing against people at their own level.
- Skill in magic and martial might is logarithmic with time at the highest levels. That means that while your apprentice starts off as relatively incompetent — say, 1/10 as powerful as you — over the course of just a couple of years of training, they will likely grow in power by an order of magnitude, while you only get 10% better. That means that they become useful, loyal, and reliable allies, and an adventurer can always use more of those.
- Showing off is fun and cathartic. Yes, of course, a low-level dungeon of spiders and rats is no challenge for you... but you remember those days when it was a struggle, and golly, it's a blast to just blow through them, and the adulation from your apprentice/s is very pleasant.
- Low-level dungeons are a safe playground to try new things. Are you going to test your new sword-maneuver or chromatic-kill-spell on a big bad dragon? What if it doesn't work? Nah, much more practical to just run a low-level dungeon with some apprentices riding along, test your new stuff there, and soften up the challenge rating for the apprentices while you're at it.
- A sense of responsibility. You were once a neophyte, but someone took you in, taught you, and helped you out. if that's something you value, it's your duty to do the same for others. As an adventurer, you are proud to continue that tradition.
As an addition to Daniel B's answer: for control over the adventurers.
Guilds like a merchant guild are tools to excert power. If merchants from another area can sell the same products but cheaper then you lose all your trade and craftsmen, so your guild can levi taxes ("permits to stand on the market with X goods costs more") on those cheaper products to stay in business or they can even disallow certain merchants from selling stuff altogether. Merchant guilds, like the one they had in my city, can also do quality checks. To maintain the standard of the cloth produced here they had set up a large scale system complete with seals that proved your products of sufficient quality to be sold and maintain the city's reputation.
However you arent talking merchants, you are talking mercenaries. And local citizenry will be glad to know that an adventurer capable of killing a firebreathing dragon is being checked upon and not murdering and stealing from them without checks. Guilds are likely set up by the (local or overarching) government.
An adventurer will have to report to an adventuring guild, even a small one. If you fail to do that the guild can report this to other guilds. If bad things happen like theft and murder and you might be the culprit the guilds can first take away the lucrative tasks for you to do. If they know who did it they can put out bounties on your head, giving even small guilds a lot of power over much more powerful adventurers.
Guilds would naturally not just check on adventurers, they serve a purpose for the overall population. From pests, monster problems, bandits and fetch quests the guild will gather the requests and set a bounty (paid by through taxes and the person/organization who sets is). But you dont want your adventurers to be dying in droves as that cuts in the local safety and profits, so you add a training program. Veterans have to help train the new generations of adventurers.
To make sure this happens the veterans will get incentives: they get paid to train the new adventurers and any more lucrative contracts these veterans might want could require a minimum of trainees passing tests and contracts of their own. This means that a good trainee who passes a lot of good contracts and bounties could weigh in more than someone who trains dozens of trainees most of which die anyway on their first solo outings. Giving extra incentives to train them well.
White Fang wants Saitama to join his dojo. As sensei, Bang-san would then be able to take credit for Saitama's superhuman feats.
Thus is it with you, the aging adventurer. You spend a lot of time hanging around with compadres, talking smack. Victories from the glory days have weathered many a brag, and new brags are welcome. What better to brag about than your student / protege, and how she is more successful in almost every way (thanks to your tutelage) than your compadre's mediocre students? Not more successful in accruing goblin bite marks, but you concede that has always been your compadre's strong suit and so his students too.
Helping the tutorial with high level quests
You can link specific high level quests with tutorials. The quests are side by side or possibly even overlapping, in which they need to help the new person while surviving the quest (optional in case you don't want to run into danger).
Either side by side you'll do the quest, or the quest involves ghost like apparition that can only interact with certain levels in the same spaces. This means that the high level will encounter high level monsters and will get high level rewards, while the low level will encounter low level monsters and get low level rewards. Linking them so the high level can't advance until the low level has cleared it's tutorial will help the pacing.
Alternatively Incentives to help are easy to come by. Especially the lower ranks can be forced to do chores or lose a rank or similar, or can't advance in the ranks without doing their chores. But even at higher levels this is the case. As an example, at the university, you can be a highly reputed scholar doing breaking research, but the next day you'll be teaching the students. It is expected and required.
Otherwise when the next opportunity comes along, like a group quest, guild decision or similar, you can be left out.
Credit for this answer goes to a book series I have recently finished, called the Unwanted Undead adventurer, where a similar theme is explored.
A quick rundown, the MC has been an adventurer for more than 10 years and is considered as extremely valuable to the guild for his extensive knowledge in different fields such as herb picking, equipment selection, party composition etc. In his free time when he is not adventuring, he often arranges free lectures at the local guild, available for anyone to join. Sometimes he gives the course, in areas that he has experience, such how to differentiate between different herbs, how to harvest them and store them etc. Sometimes he gets his mage friend to give a lecture on unusual uses of magic, and sometimes someone else gives a lecture on how to correctly harvest bits of of a poisonous creepy thing.
As the story goes on, it is established that the guild intended to hire the MC in the event that he decides to retire due to age or injury, as knowledge is and always will be a valuable commodity.
Consider that adventuring is an extremely dangerous career, not only for noobs, but even the strongest adventurer who is capable of slaying multiple dragons might slip in some slime juice while trying to side step a goblins attack. A nice cushy job training some new kids how to hide behind their shield when being attacked on the other hand is much easier and less stressful.
There is also the demand for such a high level adventurer to consider. How often do cataclysmic events occur that require the skills of someone who is at the peak of power? If dragons only attack a town once every 10 years, that is a long time where the person is simply not needed, and he will have to go kill some goblins to make money. In which case 2 things will happen; .1 the adventurer becomes depressed because these worthless monsters and tasks provide no challenge - ie. he is now worthless, and .2 a goblin that is killed by some noob, and a goblin that is killed by the great hero of townsville are both still just dead goblins, so why would someone pay more for a quest if the hero takes it - ie. no one remembers that time years ago when I killed a dragon, no one is grateful or appreciative of what I do boohoo
As such, it not only benefits the guild to provide a nice job and a stable source of income for their veteran adventures - with new generations becoming better and better, it also benefits the adventurers by keeping them relevant and feeling important.
The incentive I suggest then is a nice job assured for when they decide they have had enough of that dangerous life. If someone who is still actively adventuring wants to help out, by all means they can, and get a bit of extra pay for lectures, demonstrations and so on, but the main teachers will be the veterans who have had enough fun.
The guild needs an endless supply of new adventurers. Adventurers retire, switch jobs, or die. Especially die.
If it's a typical group of people, the vast majority will assume someone else will do it.
Therefore, the officers of the guild, whether foresightful or of an age to exempt themselves from their own requirements, force them to do so. Depending on how much it takes, it could range from discounts on dues through perks to simply being a requirement (perhaps with alternatives to it, such as serving as an officer).