Dungeon Style ; Players with a System

MC is 80 something; takes on a surprise dungeon burst that was a special rank; defeats it to save the nursing home he'd been in and regains his youth with a blessing where he'll never age, a near immortality. He'd been out of touch with the world and finds an S-Rank he'd known, hides his rank and starts over in a sense while trying to fix the association that has become corrupt in his absense.

It's a cute one, novel says it ends happy!