i think when you have your child play summer/travel lacrosse is a function of your child and how interested they are in the game and how much they want to play. I have three boys, and two of them wanted to play 5th grade Summer lax and they both loved it. I would encourage multiple sports if your child wants to do that to avoid burn out and to develop different/complementary skill sets - basketball and lax go well together; as does hockey and lax for example. | 
