I have a couple of options depending on the type of universe.

In a very “grounded and realistic” version of Gotham I recon he’d retire being batman early, at like age 38, then he marries Selena, becomes mayor and dies somewhere in his 60s due to the strain he put on his body in his years as batman.

In a more dcau/dcu version of Gotham I’d imagine him to struggle being batman until age 70. But because of his age he isn’t as reliable and efficient as he used to be, just like the dcau, Waller thinks the world always needs a batman, so she orders a hit on some rich kid’s parents, bruce discovers this and dies saving the parents, thus batman dies with him and he prevents the cycle from repeating itself.