I always felt the Joker was a character who completely transcended the concept of "age" in both how he looked and acted. With almost any other character I can place a fairly definitive age on them, and have the character work in that bracket, but it seems that the Joker defies the human psyche to such an extent that giving him an "age" almost breaks the character.
When bringing him to film though, I think a 25-35 Joker is the way to go, not only for the energy level that that type of actor can provide, but also simply because it puts Joker on the same level with Batman, which is integral to the character in my opinion. Ones of those things where two characters, who at their very barest are the same, but once you build on top of that skeleton, see that they go in entirely opposite directions.