While obviously they're not meaning to follow this up with any direct Batman sequels and it's not important, I don't think their age difference is that outlandish when playing around with the idea.
Joaquin is 45. His age isn't necessarily exactly the same as the character. Could we say that his character is a rough 40 from a hard upbringing? We know this dude has some mileage on him. (I dont remember if any hard age was established in the movie)
Dante is ~10. Could we say Bruce is 11, or possibly stretch to 12?
Bruce usually becomes Batman somewhere around age 22ish depending on the continuity, sometimes earlier sometimes later. Lets lean earlier and go with 21.
So believably, these two could clash at ages 21 and 49. Batman could always beat Joker to a pulp in a straight fight, so there's no change there. Dude would have to pull his punches which plays right into their dynamic anyway. And a 49 year old Joker isn't so old that he can't be manipulative, crazy and scheming. That's all in the head. So... what's the difference?
Those two could clash for 10 years and the Joker wouldn't yet be out of his 50s. Nolan Batman was active for less total time than that. If Tom Cruise can do what he does at 57, make-believe character Arthur Fleck could remain perfectly slippery at 49.
Their future together is still perfectly viable and that imaginary sequel would then take place in 1990.