In the movie Batman (1989), Jack Napier/The Joker is shown to have killed Bruce Wayne's parents, Thomas and Martha Wayne. Jack Napier looks like he would have been in his mid-20s or early 30s during the shooting. And Bruce would have been around 8-12 years of age. So later on, when Bruce eventually becomes the Batman fights the Joker, wouldn't their age gap be a lot larger than what's portrayed by Michael Keaton and Jack Nicholson?
Jack Nicholson and Michael Keaton are 14 years apart. Remember that Gabrielle Carteris was 21 when she first started filming Beverly Hills 90210, when she was supposed to be 17-ish in age for the role. Assuming Joker was in his early 20's when he killed Bruce's parents, and Bruce was roughly 8, you're talking about maybe a 5 year difference in Nicholson's actual age vs the age he portrayed Joker to be? Definitely plausible.
So, to answer the question, Batman would be mid-to-late 30's and Joker would be about 50. That's pretty much what it looked like on screen.