@Rules you forget that at the time that Jesus walked the earth John the Baptist just started with baptising people.
furthermore billions of people (in other denominations that consider themselves christian) are baptised right after they are born
Anyway the main reason is,... WHY NOT baptise Teens? We let them make career and education choices that influence them the rest of their lifes at that age so why not the choice to serve Jehovah?
there are biblical examples of young people who make a stand for Jehovah,.. Samuel did, David did,.. Jesus did, to name a few