Well I use a simple formula 30 and younger for a 19 year old or less. Once they hit 20 it's pretty much anyone who will take her under 45 years. The shorter you are in family members the more of them will be young. If I'm close to 1 general per city I may except greater age differences as not too many opportunties present themselves in this case.

Also for those that don't know how to tell who's marrying who, heres how.

You get the message 35 year old Cornelius Scipio wants to marry Fastia. First go to your factions folder, it will automatically minimize the marriage screen. Open your family tree and look for Fastia, you can see her age and if the her age compared to the husbands age is suitable to you tastes you can either accept it or turn it down.

Another thing daughters are a bit preprogrammed from my observation. I noticed this when I ran into a minor bug with one of the husbands. I went back a few turns and the same husband's kept showing up in the exact order. I believe they had some kind of feature in there with possibly civil wars but changed it so that it could only happen to the Romans and did'nt use family members to provoke it.