Obviously there are always exceptions to the rule but I think it's worth pointing out that of the 20 outfield players England have selected for this World Cup (keepers are a bit different) 12 of them made their England debut at 21 or younger. I think that shows that if you've not even made it into your club side by that age, you may well not make the step up.
Of the other 8, 3 had 'unconventional' paths to the national side, in that they were seemingly late bloomers who made their way up through the leagues, so only 5 of the 20 were at PL clubs but not playing for England at 21.
This is why I find it odd sometimes that people make out like DCL, Holgate and Kenny can't be expected to perform at a reasonable level in the PL yet but are destined for greatness. The fact is, most of the players who make it at the top are well on their way there at 21.
When did Gerrard breakthrough at Liverpool?
Edit: Ignore that he was 18. Swear I thought he was after 20 or was that Murphy.