In other words, whatever the stats of the king the year the prince matures, that is the base for the stats of the prince. Keeping this in mind it could be worthwhile to put an attack with your king as general off for a few years if this king has a good attacker virtue until your prince is 15.
I do this regulary playing as the Volga-Bulgars. My Khan has 3 stars but the good attacker virtue is easy to win in the opening years. Timing my attacks just right, means that when the heir matures, my king is just on the offensive and facing a field battle with some rebels. 3 stars become 4 (vitrue) and my heir will have better stats than he would have gotten with a 3 star father.