It's about the future.
Today, the difference is what you can see, one player is a little better.
But tomorrow if you train your players, the 20 year old will improve a lot faster than the 30 year old. After only a few days of hard training, the 20 year old can be as good as the 30 year old.
Fast forward to next season, and the 31 year old is still not going to improve much, but the 21 year old could be a lot better. When the 30+ year old retires, the 20+ year old could be your star player.
Does that help?