Take a look at TJ's
age duration hack. He outlined what was being done to certain age groups, but the mod serves as an excellent way to learn adjusting the values for however you want for each age group.
The values TJ assigned to the mod seemed realistic enough for me. The only age group that's not affected by the mod is the Young Adult group whose aging process is stopped once they go to college. So they're excluded. BUT if you want them to be Young Adults for longer than the normal Maxis semesters (1 Semester = 3 days), you can alter the values to a shorter college stint or a longer one for your YA. I think that part is in a different BCON, but it shouldn't be hard to find with SimPE. Someone at MTS did post a couple flavors to shorten or lengthen the college year. I don't remember the name right off the top of my head, but you should be able to locate that without a problem if you want to adjust the values for the YA group.