When people talk about why 'depth' is an advantage for a team, I think sometimes the point is missed about why.
The advantage of having quality depth isn't really so much about avoiding 20 year old college guys getting too tired during a game. They are 20 year olds.
The advantages are much more about increasing the quality of the practices (like Bobinator mentioned), increasing the competition for playing time, and then of course during games, makes it easier to absorb foul trouble or injuries.