I think very few players are going to come through with the top counties now before they are 23. In an amateur context, you probably can't build the physique fast enough. There will be exceptions, but these will mainly be guys with some inbuilt physical advantage (strength, speed etc.).
There is lots of talk about guys retiring early these days because of the demands of inter county. But in my opinion this always happened. Pre the days when the majority of youngsters went to college, I know some serious prospects who retired after minor because they needed to work.
I think the reality in the future isn't younger players, it will actually be older players. They will start later and stay longer. For the top teams anyway, the rewards are decent if handled the right way. An inter county player can get away with playing 10 to 15 games a year. If training is handled correctly, he can have very low mileage at 35, 36.