IMHO a 135 is as big as I would go for a 13 year old. My oldest son started at 10 on a Liquid Force Nemesis 124 and this summer upgraded to a Liquid Force Harley 131 at 13. The Harley 131 is perfect for him and will provide lots of growing room. He is now 13 and 120 lbs. I think the weight of anything bigger than a 135 will be tough until he has the strength to handle it. My 11 Year old has taken over the Nemesis 124 and it fits him well. My experience with them is the smaller (lighter) the board, the easier it is for them to learn to handle and is much more fun. We have loved the Liquid Force boards.
On the 131 just after upgrading:
On the 124 just before upgrading:
Good luck and hope you all have as much fun with your kids and their wakeboarding as we do.