I currently play with the idea of a story where the protagonist travels a foreign world, visits places, studies customs hand history of locals, meets with survivors of historical events and takes a look at the physical characteristics and biology of the inhabitants and also their technology. It's basically like a fictional documentary and the protagonist is doing the research and narrating it.
The dominant life form of that world can be described as being basically dragons (the protagonist is of an other non human species). They look like you would expect a standard dragon to look like (four legs + a pair of wings). An adult dragon usually has a shoulder height between 1.5m and 2m (depending on subspecies and age and region). They walk an all fours but do everything else on two legs, basically sitting to have their hands free. Of course they can spit fire and fly.
As mentioned the Dragons are physically mostly what you would expect. They do look like reptiles (although some have birdlike feathers) but they are warm blooded and have reproductive organs that resemble much more those of a mammal. The Dragons are basically ovovivipary. Their young one is born alive except when environmental conditions aren't good e.g. there is not enough food or the female gets weak or sick. They can't actively control it but it could be induced by medication i.e by taking hormones. In this case the female lays eggs which shortens the pregnancy a lot. Most Dragons do not feed milk to the newborns.
The dragons are extremely long-living (500+ years are normal and they usually die by accident or sickness), robust, and not very fertile. Some female may engage in sexual activities regularly over 500 Years and never get pregnant but the opposite case also exists. Some females are very fertile and often act as surrogate mothers for other dragons that have no luck with getting a child by themselves. Artificial insemination almost never works. The fertility issue is subject to research but the cause or a cure is still not found. According to their research almost every female should have no problem getting pregnant. The fertility issue over was incorporated in the dragnon's main religion.
The average birth rate is slightly higher then the death rate. Population grows steadily but slowly and is, at the time when the protagonist visits their home world, at about seven milliards, split across several worlds an colonies.
If the pregnancy is successful the female gives birth to only one child (or lays one egg) at a time. Twins are extremely rare and usually reason for a big celebration. Complications during pregnancy or birth are very rare nowadays.
The young ones mature very quickly and grow fast in their first few years and then slowly grow to their full size over many years.
Most dragons have in common that they're curios and playful. They love games and riddles, in fact, in their home world they are so obsessed with games that they even settle major conflicts with games and sports.
The dragons, despite being technologically very advanced (colonizing space and have interdimensional travel), not only still look very feral, they value their feral roots and instincts as well, instinct driven behavior still plays an important role in their society and social interactions.
The young ones enter the school system at age four and visit school for about six years. Between the age 10 and 12 they usually get sexually mature. At the age of 10, they are allowed to move out, it's also the age where they're granted their first adult privileges, although not technically adult by the law (they are now young adults). They can now choose to enter a secondary school and study, get a job (usually apprenticeship) or travel the world before entering a career (which is tradition often followed). They can also get involved into some levels of Politics (like regional voting or joining a party).
The dragons live (at the time the protagonist visits them) in a post-scarcity society (similar to Earth in Star Trek although not that perfect). Medical science is very advanced and early pregnancy wouldn't bear much risk, STDs are rare and curable.
They have a tight social net of benefits that would support a young mother/father (males and females are treated equal in any way) in need and many dragons, that are unlucky with getting offspring by themselves, would like to adopt a child. Generally, rising a young dragon is often not done by a single pair of parents. Instead it's more a group effort mostly because most relationships are polygamous. Relationships are usually short.
Life long bonds are rare and marriage is a foreign concept although it happens, is influenced by other races and cultures, and usually only done in interspecies relationships with races that have shorter lifespans. Interspecies sex happens too, but there are no reports of a female dragon getting impregnated by a partner of a non-dragon race while males seem to be able to reproduce with some non-dragon races. In that case the offspring is always of the mothers race, so dragon-male + non-dragon-female = non-dragon offsping.
Dragons utilize inter-dimensional travel and colonize other "planes" instead of relying very much on space travel. They have contacts to many other species (also visited our earth at some point) some are just as advanced.
Sexual interaction is mostly not for procreation but more for the fun. Contraception is effective but usually not seen as needed since the dragons are very unfertile anyway. Therefore "teen" pregnancies are very rare too.
Why could sex still be not allowed for the young adults?
From my design of the race and its society I don't see why it shouldn't be perfectly okay for the young dragons to engage in sexual activities. It fits a race that gives in to it's instincts, like the mating instinct, where pregnancy and disease is a minimal risk and where sex could be seem to them as a form of play.
I want the dragon race to be a bit more relatable to human standards and want them to have good reasons why they forbid or at least strongly disprove that young dragons engage in sexual activities with each other and with older dragons as well.
I first thought of the dragon's courtship ritual maybe being dangerous and violent (like Klingon mating habits), therefore making it risky although everything else is not a problem. But I rather like my dragons to be nonviolent in any way that is not fighting in war or something like martial arts in sports.