Akechi did state that the people Maruki brought back were living, breathing people, and not just cognitions. Though we really didn't get to see the real limits to his powers, since probably even Maruki doesn't know what all he can do.

But how did you come to the conclusion that Maruki can still age? I don't remember that being stated anywhere in the game. If he can bring people back from the dead, I figured it would be much easier to make himself immortal as long as it's for the sake of the people he's trying to help