There is actually nothing wrong with holdbacks. If someone wants to be 18 and a sophomore in HS, fine. But youth lacrosse should be organized by birth year, Hockey is a great example of how this can work. There, unless a kid is a Jack Eichel type phenom, the college coaches don't want him at 18 when he leaves HS. The large majority of freshman playing D1 hockey are 19-20 yo. For on age kids, the kids will play a year or two in juniors (or a top level 18U team) until they are ready.
Thus, club lacrosse should be age based through age 15. Starting at 16, there should be a junior system where the top 16-20 year olds not yet in college play - this should be a closed invite only league with only elite players. For every one else - it will continue age based through 18.