In some ways that is actually normal. People get wiser as they age more (usually, anyway), therefore they have a higher education than the younger folks. If you've had a great education system for a while, the older population should have excellent education levels, while the young ones are still learning.
Having lots of libraries is good to have, as would be some city ordinances for improving education, if you haven't done that already. It may also just take some time for things to balance out nicely.
