How would you accommodate school districts who's cutoff date is before 9/1? This is the bigger issue if you're trying to address all captured players. Rules that allow X number of players to play down won't work when 18 players that are playing down eligable are on a team from a district with an earlier school start date.
A way to get around this would be to provide different districts different numbers of players that can play down. But you can see how this would get hard to maintain
You must choose between accommodating all trap players or maintaining the game's fairness. You can not have both. So between accommodating a very few percentage of trap players left for whatever reason after the switch or be fair to the other 95% players. I think the 12-month range is the maximum to keep the youngest players still in the game. I agree with ECNL 8/1 or 9/1 cut off for 12 months age group with No wavier.