Re: Historical use of Roman generals and governors
What you're doing sounds fine.
A 20 year-old governor is not ideal - but given the choice between a young governor and no governor at all (because of a shortage of family members) a young governor is better.
I would assign governor positions to family members in order of age seniority, however - oldest available man first, then the next oldest, and so on. Even if that means a more competent young man has to wait his turn for a governor's position to become available.