In the middle ages the bitch was probrably over the hill at 16
. Somehere around 16 would be fine, you would not want some under developed bitch anyway. This way 18 year olds aren't breaking the law either dating (fucking) anyone younger, it would also eliminate state variation or the 16 to 24 with consent shit you hear from time to time in some states.
I see what you mean about societys laws, but I don't think there are too many sub 16 year olds that have what I would call fully developed attractive bodies but to each his own.