Background Info: All human beings possess magic in some way, shape, or form. This magic can express itself in a variety of ways, which is where the different types of magic-users come into play (shaman, witch, warlock, sorcerer, enchantress, wizard, etc.), but there are some passive effects that affect all humans in roughly the same fashion regardless of what type of magic they have.
The main thing I'm focusing on is the fact that the stronger your magic, the longer your lifespan, and the strongest magic-users just age into their prime and stay there. They can still die from disease (especially magical diseases), accidents, violence, and so on, but they don't have to worry about something as silly as old age getting in the way of their lives.
The amount of power someone possesses is usually decided by how powerful the parents are. There are exceptions of course (atavism, random chance, and ultra-recessive genes have an impact) but this is usually how it works. Now, the stronger your magic, the less fertile you are. A natural way to keep population numbers from exploding. This is a good thing, since without a natural die-off, the immortal population might soon outnumber those who simply age slower.
What I'm looking for are some of the societal implications of having an ultra-powerful, immortal 1% of the population. Usually families, since power is genetic.
I know that these families will likely be ridiculously wealthy, since even someone paid minimum wage could become a billionaire if they live frugally and give it enough time, let alone those with immensely powerful magic and the time to build up lucrative businesses.
I also know that they'll have great social influence; if someone has been a prominent member of society from the day that your great-great-grandmother was born, they're going to be viewed as an unchanging pillar of the social landscape by the masses, and their opinions will be viewed as important simply because of how much experience they have.
These families would also probably have arranged marriages, because power is genetic, and they want their descendants to be powerful (to contribute to the family) and live long lives (nobody wants to see their children wither away from old age while they remain in their prime). New blood in the form of random occurrences of immortals in the general population would be enough to prevent inbreeding, like was a problem for the aristocracy in Europe. After all, they aren't marrying for alliances and wealth (although that does play a part)- they are marrying for power.
Inheritance disputes would probably be an issue. Some people are power hungry enough that they won't be willing to wait for those above them in the hierarchy to die off from attrition- they'll try and engineer the deaths of their parents, grandparents, and so on so that they can rise up the ranks.
These are the main things I've figured out. I want to know if I've missed anything.