We got this question on the 2012 Worlds FB page that I couldn't answer:

If my son's birthday is in May 1998 and he is currently 13 but will be 14 by July what division does he play in? MJ3 or MJ2.

The division is based on the oldest age the Junior will be during the calendar year. In this case, he would have to play MJ2 for boys 16 and under during 2012. See page 4 of Tour Standards for confirmation: http://www.pdga.com/files/documents/2012_PDGA_Tour_Standards_v2.pdf