The two year time frame is due to the possibility that the foam/gel material will start to breakdown, either from use or the naturally. The statement is there just in case something happens you can't go back and sue them. The same thing applies to motorcycle helmets. The frame and shell should not have any issues.
My DD is only 9 going on 10 and just got her new gear. We went with All-Star Vela chest protector and leg guards and the MVP 2510 helmet because it fit her best. She said everything else felt either too bulky or too tight. Since I know she will be going through some growth spurts I figure I will be changing her gear every time she moves up an age bracket. Sometimes it may be more often, hopefully not.