Both have shown enough, especially with the ball that it is not age but ability holding them back from being top level PL players.
The decisions that Dier makes with the ball at his feet is not because he is 27, it is because he absolutely sucks with the ball at his feet. I don't think top level teams can get by with CB's that are terrible at playing the ball. They don't all have to be world class but they have to be comfortable or it puts too much pressure on the other players and it is too easy to pressure that player to cause major trouble.
Sanchez has also shown that he needs to be protected by a better CB to limit how much he has to do. Again I don't think you can expect to compete at the top 4 level with a CB that can't be expected to think the game for himself.
That is not because of who is in front of them, that is clear limitations in their game that puts them into trouble all on their own. Yes improving the guys in front of them would limit the chances they have to screw it up but they still have those major flaws and that is not something you can get away with in your CB's if you want to be competitive.