Who, where?
It's nonsense.
Stamina, power, explosiveness, reaction times, recovery times, speed -- quite literally everything that matters in sports (except endurance/strength -- to an extent) starts getting worse in early/mid twenties. Experience can help suppress the inevitable deterioration stemming from aging in the late twenties, but only players who didn't work hard enough to fulfill their peak potential in their twenties can be better in early thirties or mid thirties than they were at their (potential) physical peak, and that's far from the case with Big 3.
By the time you hit the mid thirties, your reflexes have already noticeably dulled out compared with your peak. That in itself is a huge detriment for a tennis pro.
I'm sorry, but I have watched Federer play in 2005 and 2006. People suggesting he was better in 2009 are out to lunch, let alone 2017.