I think a very simple reason may be because the very young are pretty much the only ones naive enough to take the plunge these days. With the internet available to inform people about the deceptive lies printed by the WT corporation it seems that they are getting their member from the less informed population like the elderly and the very young. They/GB hate higher education because they loose members who get one and so they are focusing on those that are dumb enough to believe their boogie man bull shit about Jehovah choosing their corporation and anybody not a part of it he will destroyed at armageddon.
Also we need to consider that they view these young people as a potential free labor source so the sooner you get them hooked into baptism with the threat of disfellowshipping you can get more free labor out of them.