@BattleFranky69:
Maybe it's because it's too easy? You think about a person who's really old, you kind of expect them to be cynical after a lifetime of shit and so obviously, boom, psycho supervillain. Having someone become that at a young age, especially from a very, very young age like when Bruce Wayne experiences his trauma that drives him to become Batman, makes a more appropriate mirror of the Dark Knight. At least, if you forego the whole 'falls into a vat of chemicals' backstory.
Jack Nicolson wasn't exactly doing acrobatics.
Until further notice, this is a stand-alone movie. It doesn't matter. I've read people people call this, "A batman movie", or they overthink it in terms of its placing in a Batman's mythology. It's its own thing. Let Joker be 20, let him be 70. His age isn't his defining feature, nor is his backstory, a version of which I think The Dark Knight did brilliantly. Falling into a pool of chemicals that die your hair green wouldn't really have fit.