Federer wins grand slam titles at the age of 37
Granted Tennis doesn't particularly require the same level of endurance as football but i don't know, it's as much to do with fitness as its to do with will and hunger(which Cristiano has plenty). Once he starts to lose his athleticism , he will remodel himself AGAIN where he'd be as effective as before or atleast close to it.
Zlatan at 35 was destroying the high intensity EPL and I don't think he has the same level of dedication and hunger as Cristiano does given his talent.