SHARK, I have made different assumptions as to the incidence of excellence in the general population of my gameworlds. Given that the world is usually crawling with nasty things that want to eat your face, fighters and thieves would have ample chances to train, regardless of where they happened to be. Clerics and magic-users, however, needed specialized training areas to even begin to learn their class. Given literature and the number of "failed wizard experiments" there are in the Monster Manual, I furthermore decided that magic-users were fairly unpopular. Those who had the talent to access magic would become clerics if they could.
Therefore, it would not seem to be out of line for a good-sized, prosperous villiage of 500+ to have a small temple that had a single level 1 cleric and a couple of acolytes that were devout, but unpowered. He might have some scrolls given to him by the Mother Church, or even some passed down from a particularlly capable abbott. Assuming that the cleric is benevolent, or at least wants to get in good with the populace, he would offer his services to the people.
I figured that the presence of magic balanced out the presence of monsters, survivability wise. It did skew the population, however. Having a cleric, presumably trained in the healing arts as well as doctine and magic, would make an exceptional midwife. Improving infant and maternal survivability would go a long way to increasing the stability of a village or town. Even using a rare cure disease scroll for childbirth fever would be appropriate. But, he's just one man (or woman). The presence of a plague or other serious situation he just gets overwhelmed. Either because he doesn't have the scrolls to cure the infected or he can't catch up if he can memorize the spell.
So, my upshot was that the 5 year mortality rate would be significantly, if not drastically, less. All-cause mortality for 16-30 year olds would be higher, than historical because of the ankhegs, manticores and carrion crawlers that jump out at you when you are in the fields. Those that made it to 50 and 70 years old would be about the same.
And since so few made it to a magical apprenticeship, there isn't the number of magicians to even conceive of a mercantile for magic. There certainly is a market for magic items, but I figured that it would be parallel to the current market for high end jewelry. Done by the rich, lucky or clever in comfortable hidden areas that the general population doesn't know about. You can find it if you look, but only those that can affort it do so.