Many jobs in Tropico have a minimum age limit. Even if the only alternative is unemployment, a Tropican may not take this job until they reach the minimum age. They will not even start school to fill an open job until they meet the age requirement.
18 journalists, policeman, teachers, barmaids, showgirls
21 bishops, doctors, bankers, professors, generals, priests, pitbosses
While the latter portion of your post is mostly right, this portion is dead wrong.
The only requirements for a particular job are:
1) Acceptable faction leaning
2) Sufficient education level
3) Over age 13
I've had doctors who were 17 years old. They started school at age 13 and stuck with it all the way through college. Of course, my school and college was fully staffed which made for fast schooling.