Well, some times, someone ask a simple question, and receive in exchange several absurd replies. When there are no good answers, a new question is the answer. Very ugly. There are a lot of reasons to change the birthdate for an avatar. By example you are going to use an avatar who was from your brother (who have less than 18) and have invested a lot of L$ or time with it: Reply: "Trow your bro avatar to trash, and build one new fresh." Is stupid. But continue, is very easy, let the users get tired for nosenses, and you will open the big door to empty SL .