Interesting ideas there Irda Ranger, but I disagree with a few of your assumptions.
Firstly, an even spread among the ages. This just feels plain wrong to me, as your assumption here is that no-one dies of anything other than old age. Given the high attrition rate of deaths from red wizard training, infighting and wars alone, I think that this is a bad assumption to make. There should be a significant drop in the number of oldies around compared to the young, but I am not sure about how much the attrition rate should be.
As for the rate of leveling up, two points.
First, Red Wizards are famous for selling magic items. All those magic items cost xp to make, so that could significantly slow the rate of advancement of a red wizard. After all, if you earn enough xp to level once every three years, how many xp could you have blown into magic items in that time?
Secondly, you are assuming that a wizard will get those 4 to 5 encounters of a challenge rating equal to his level per year. I think that the number of higher CR encounters should be significantly lower for a non-adventuring wizard, slowing down the rate of advancement considerably at higher levels. Heck, the average challenge rating they will probably face would be about CR 1 (they aren't adventurers after all), so they shouldn't get xp above 9th level for them. On the flip side however, infighting should provide greater xp (other red wizards that are a threat to you probably have a CR about your level), but this would also increase the attrition rate even further.