Yes, the very best young players are usually kept by their clubs. I'm of the opinion that clubs loan out players as a way to judge if they will make it as professionals. It's virtually impossible to tell if, say, an average 18 year old on the books at Chelsea will make the grade; some are late developers, some regress, some can't handle the pressure. Clubs loan out players as a way to keep them on the books on the off chance they will become stars. It would be embarrassing releasing an 18 year old if he somehow turned into a £50 million player as few years later. We know that most of these 18 year olds will at best turn out to be journeyman pros, but NO-ONE can really tell which 18 year old will turn out to be the next Messi and which ones will turn out to be the next Nicky Law.