he are some reasons which lead me to believe him to be older than bats.....The joker was a low level criminal for sometime well before bruce wayne ever became batman, than he became redhood and the rest is history, but he it has been implyed that he was a low-level criminal at sometime, that really doesn't give you as much indication of his age as Alan Moore's version in which the Joker already had a engineer degree, a wife, and a son on the way, and once again this is before bruce ever became batman....in this description alone is sounds like the joker is a good 5 years ahead of batman, so maybe not mid 40s but atleast early 40s
Interesting topic....I've always consider Bruce and Selina to be in their early 30's. Same with Ivy, Talia and Harvey. Joker is somewhere around his mid 30's. Harley is no doubt in between 21-24. Jason Todd must be 27.
The Older characters:
Mr Freeze is 46 he was in colleage around when of Thomas Wayne was still alive. Hugo Strange in his late 40's. Ra is ancient fossil and Penguin is 44. Riddler must be 40 years old.