I'm with Dom when he says the name Robin was just in-joke nod. It's made very clear that Blake is supposed to takeover as Batman. It also made it very clear that Bruce couldn't keep being Batman anyway. Remember that scene where the doctor told him he'd worn away all his cartilage? And how the only way he was able to get back in the fight was by wearing those special bracers? Those are just a temporary solution, he can't go on indefinitely in that condition. Therefore he has to retire because his body is just too worn out. It's not the years it's the mileage.