That works for football, because of its very short, defined season. Chances are a kid won't outgrow his team over a couple of months. Lacrosse, which is now a year round sport, makes that solution impractical, because of the challenges of having to continuously replace players mid-season, as the grow beyond weight/height limits. I think parents and kids are ok playing against bigger kids as long as they are the same age. I agree with previous posters, that even if a sixth and eight grader are the same height and weight, muscle development in the eight grader will typically make him stronger and quicker.