John Cena is 38 years old. He has not even hit 40 yet. And as long as he doesn't get some real serious injuries he can keep working for a good while yet. Jerry Lawler still wrestles on the independents and does a fine job of it. Especially if John saunters down in the card and works with rookies once his time on top has passed.
Here's another example of a wrestler working well into old age.
This, is Masanobu Fuchi:
He's 61 years old and still works actively for All Japan pro wrestling. 61 years old. He has adapted his style to fit his age and works more of a comedy based style than his old martial arts and high flying based style. But he still does work to this day.
If John Cena wants to work until he is 60, he can.