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28 is too young. One thing I have a problem with is how there is so much emphasis on the character the Joker as being the cause of his mental problem. I've seen actors take on high quality characters. And have fun in playing them. Personally I feel that his problem began well before the Joker.
In fact, I think he took on the Joker because he thought that taking on a character like that would make him feel more alive or something. But when he found that it didn't do that for him that's when his already depression gets worst.
That is what I think happened in regards to the Joker. In fact, I would say the name matched up with what he felt he got instead of what he wanted. And in dwelling on that he drops further.
For the first time in Batman history the Joker is the innocent by stander.