I'm of the opinion that if you're good enough, you're old enough. Players should be selected based on their ability to perform in the team and little else. Arbitrary factors like age, height, weight etc should all be secondary to their actual performance. Mercer again always sticks out for me, I was on his bandwagon back when he was in U20's and the common criticism was he was too small and wouldn't be able to handle bigger guys. Nothing he did changed that perception until he went to France and was excelling against some of the biggest and best players in the world. Even then his size and weight were brought up, despite mountains of evidence it counted for sweet **** all on the field.
I'm tired of the apparent coaching by spreadsheet mentality we seem to have in England (club and country), if players don't tick the right boxes that statistics say need to be ticked, they never really get a proper chance no matter how they perform. Meanwhile someone that does tick the boxes but actually performs poorly stays in because spreadsheet says so.
I do feel a lot of England coaches don't even watch the games and go over footage, instead just pouring through tons of stats.