Just something I was thinking about.
When I was 12 and 13, I used to make all my characters 16 to 18 years old. I thought they were then old and tough. Of course, I now think that's young and inexperienced ;).
Lately, I make my fighters in their late teens or early 20's, because honestly, that is a great starting age for a beginning warrior. I make my cleric and mage characters either late 20's all the way through early 40's. It just makes more sense to me, as I think most people wouldn't either have the education (or at least access to magical tomes and scrolls) or religious insight until their 30's sometime to really justify these career paths. I know that's a bit silly, but it's how I view the world now.