A way could be to tag the sl last name to TeenResident instead of just Resident and the last name change be made automagically based upon declared age, the accounts uuid remains the same but the server reports the age related lastname when queried. Such queries could be in a profile lookup or when the user interacts and the user name is obtained so it can be rendered in 3d scene or in the various chats.
My solution to the social interaction dilema is the same as I always use in all chat programs is, forget online sex... there is no 100% way to be certain what it is you are interacting with... male, female, or anything in between. Age level: minor, adult,or wrinkly... or single, married or anything in between. I personaly choose to use interactions for platonic friendship. While this won't be acceptable to the vast majority of current sl users, it's worked for me in chats where the age limits were anything above 13 were permitted... others that chose to seek cyber-sex in such virtual worlds took their own risks... and some found out their cyber-sex partner was some teen girl crushing on an older man and then had to deal with the consiquences.