Dont think its as straightforward as that. There is a reason why Wenger wont get Walcott to start for Arsenal regularly. There's a reason why Messi, too was bought slowly into the Barca squad(yes, i know, he's still only 19. but listen me out anyway). Everyone knew that these two players are going to probably be world superstars in the future but still they are only being bedded in slowly.
Some of these lads may have the quality to fight it out but not the physical structure at 17-18-19 years old. It might shatter their confidence if the dont do well, or god forbid get injured..
The others arent mentally equipped to handle the pressure of the premiership.
It wouldnt be wise to start such players unnecessarily. Huge risk.
But, if the boys (at 18, they are boys really) are both mentally AND physically equipped to handle the frustration of a Drogba dive and then physical wrestling for a long ball, or a Gerrard full blooded challenge (non-malicious though ) or a Ricardo Carvalho physical bollocking, i see no reason why the should be started.
I hope i've put my point forth, clear enough.