You are very very wrong.
Djokovic won first grand slam at the age of 20. Far earlier than Roger Federer. Second at age 23.
Zverev will be 27 before next slam rolls around.
You know first you need to be capable of achieving greatness and then see how much you can push further.
Djokovic also won 3 slams at age 23/24. He was never LATE. You are fed propaganda if you think he was late or started late. Zverev is very late. I mean he can still win a slam but should we really care.
I would not.