Balance definitely wasn't what I was looking for in the roster, but if I where to, there would probably need to be more older characters not less. After all, we have 4 milfs being quite a bit older and 8 teens all being in a somewhat comparable age range with the MC (2 a bit older, 4 about the same age, 2 a bit younger). The only thing that's really missing are probably girls in their 20s and maybe 40s+, but with the setting of the game I think that young mothers and teens come most natural.
The only perspective with which I could see it being unbalanced towards older characters is if we where to take the average of all the age gaps. But here its only natural for the average to end up a bit higher, as the milfs are the only ones that have significant age gap at all. The only way to make this number more 'balanced' would either be to have no milfs at all, add a bunch more girls with an age gap of 0, or to add girls with a significant negative age gap, which, in the setting of the game, would lead to things I'd rather not want to do.
But that's kind of a weird approach to planing a roster of Li in my opinion and in the end I don't think balance is much of a virtue of its own anyway. Variety, yes, but I like to think that the roster isn't too bad in that regard.