Too old to be at the rank of captain is the point. Him being captain wouldn't be explainable outside claiming he did something grossly out of character that got him demoted from admiral or above.
Or he declined promotions...becuase he wanted to stay as a captain. I think that would be in character for him. Or maybe he retired and was reactivated for some critical need?
And honestly we dont know that he will still be a Captain in the show. All Stewart said was "Jean Luc Picard is back".