I don't think that's how the club thinks at all. City won't care whether he breaks through at 18 or 22, only whether his final level is good enough for the first team. You don't get extra points for being precocious. He's getting experience to help his development, not because the squad is in need of his contribution.
With someone like TD there is no rush at all. He'll need more senior exposure to develop at some point, of course, but it makes no real difference to the club whether he gets that at City or elsewhere, providing he had a route into the first team of he is good enough.