Magicians aren't uncommon in my setting, but magic decays the body (mortals don't make great conduits for magical energy). The "witch mark" can take many forms: charred skin, cancerous growths, arthritic bones, infestations of insects, body parts turning to smoke, breaking apart into colors, losing your shadow. The magician typically only lasts to middle age.

That being said, a wizard is a powerful tool and coveted by the rich and politically connected. So there are tests available to the general populace, if you place well and are lucky, some noble might purchase you from you family (setting them up for a life of comfort) and send you to university. You'll be well-tended and pampered, despite that you are technically a slave.