The rule should be more about training than age. In football I believe some competitions have a rule that you have to have so many home grown players in your team. Homegrown is defined as a player that has been on a British team for at least 3 years prior to the age of 21.
The homegrown rule encourages bringing talent up. the current rule is very shortsighted and as people have said means potentially talented players will waste away on the bench for a year or so before being dumped when they turn 24.