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To answer the first part you should answer the second. XD
Since most magic requires books and/or training, middle to upper class individuals are more apt to become mages -- they can afford tutors and books that way. However, that's not always the case, and some tutors may offer their services for free. However, considering that if they have no money, they're probably spending all their time trying to survive and make what little profit they can, it only makes sense they're probably working on the farm instead of reading magic books. :P
As for how common it is, you pretty much want to remove the top and bottom end of the grand Pyramid O' Wealth, as the richest people probably have minions they can order to learn magic instead. Then, take the remaing part and split it in half, because a lot of people have at least a little magic, but a lot don't, either by preference or because they're horribly untalented. Thus, it's not unusual for people to have or not have magic (except maybe high elves, since they're more focused on the stuff).
Mainly, it depends on the character. :P
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