Age doesn't affect stamina (apart from the gradual drop in stamina that goes for all players equally), so the fact that your opponents have older players doesn't mean they have bad stamina or your young guys can outrun them. I don't think age should be the reason to go for stamina training.
That said, good stamina is important. If you have multiple important players with 3 or less stamina, you should train it a bit. If you're also training 18-21 year olds, only train it during allstar/offseason so it doesn't affect their TSP training.