Not so much as an age limit, but I have always thought that the problem with the bosman rule and with contracts nowadays is that most aren't rolling contracts. It would make much more sense to say that if a player was not selected in the clubs squad of 25, or played less than 10 or so games a season, that they should be able to move elsewhere for a nominal fee. If a player is amazing at 20, other teams should be able to sign them for a fair price.