I try and play youngsters as much as possible.
Like Hammerhead said, its much easier to train Goalkeepers because if you've got a good defence infront of them, they'll be fine.
Playing three upfront is a good way to train strikers as you can play one really good striker in the middle, and have the two youngsters playing off him. Obviously this works a lot better when you have someone like Toni, Ibrahimovich or Koller in the middle, but still a viable strategy.