There are quite a few such examples but I think Ramos is the perfect example.
At the age of 34 he gives one of the best seasons of his career and simply plays every game.
I have one more thing to say - If there was an option to bring Van Dijk to City right now, would anyone here refuse? Certainly not, probably the best CB in the world. Van Dijk and Koulibaly are the same age.
In short, Koulibaly can provide at least 3-4 years in his top and with all due respect to Garcia, regardless of the fact that I think he's overrated and he can't be a starter at City, you can't compare him to Koulibaly , especially not at the moment