Amazed to see posters wanting us to sign thiago and koulibaly. Then trying to justify the age thing. The problem with signing older players is that as soon as you have signed them you need to start planning long term replacements.
Walker , Thiago and Koulibaly would all need replacing around the same time.
When you sign younger players of the required level, they have a resale value, or stay and become legends, like Sergio , Kompany, and Silva.
Fernandinho is an exception , but for every one of those I could name a handful who fail.
Older players means constant rebuilding, rather than adding one or two players of promise and quality.
Most, if not all professionals have long term injuries that they have to put up with as they get older, Aguero, has been in pain with that knee for years, tolerance of those injuries, and susceptibility to new ones increases with age.