I don't know how difficult this is, but I think that a number of traits should be linked to the image set. e.g. A [loli] girl should look fairly young, and a young appearing image set should have low age and the [loli] trait. A [big boobs] girl should have ... big boobs. etc.
I know that 'Every slave is 17 or older' but it annoys me when a slave 'age 24' looks about age n*ne. Image sets like that shouldn't be in the default distribution if you're trying to play it save with the 'age' of girls in the game. (There's arguably plausible deniability if they are in separate downloads provided by third parties)