First of all, it's true that most of their players are getting older and they need younger players, but also I would like to point out that they are doing a bit of rebuilding while also keeping their old players as well. Gvardiol was a great transfer, only 22 years old (even though he looks 40 lol), Doku is 22 years old too, and Foden isn't a transfer, but academy player, only 24 years old, and obviously Haaland, he is 24 years old as well.
So, we have 4 players already that are under 25 years old, and if they can continue to rebuild with like one or two younger players every season, they should be fine. But that's a bit hard, because what 23 year old could play as well as KDB? Or Silva? Or most other players, it's a tough deal, they would obviously love to rebuild, but it's difficult.
Aged players are experienced players, they are always useful in a team because they guide the young talented players to the right way, but if the number of old or aged playes is much, it will affect the team’s performance because they can’t play to the required energy the team will need to have their desired result.
Manchester City squad as the subject of discussion is not as old as you may think, the important players are not as old to perform less; I think it is just time for the team to face such challenges especially as Rodri is injured. But there is a need for the management to sign new young players so that they will learn from these old players.