My son is 12 and doesn't turn 13 until spring and only throws 61-62mph, last season was the first time since 8 that his velocity didn't increase 5mph, rather it increase only by 2mph. The only exercise we eliminated was the long toss, we figured because he was on a better team that they would have better program and the extra work was not needed. This season to get ready for the spring 13U we will implement the long toss (probably late Jan.)as well as other core exercises, not to keen on heavy lifting at this age. We have never used weighted balls in any of our programs. So we are going to see how the long toss in conjunction with core and light weight leg exercise program will produce. When I say light weight, I mean 2-4lbs for the legs.