Phoenix and Ledger played the same Joker

>>Considering this film takes place in the early 80s

That's an aesthetic choice that you shouldn't be distracted by. It's Gotham looking like New York of 1981. they chose that look to mimic all the films that inspired the film from that era. In my opinion it worked well.

When it comes to age, yes, Phoenix is obviously older than Ledger, but at the end when he was full on Joker they could pass as about the same age. But it's not about age, but more about about the psychology and mentality of the character. That the Phoenix Joker will one day become the TDK Joker. At least I had fun imagining it.

With movies you have to throw out actors' ages and just imagine them younger than they really are or older than the really are, whatever is fitting to the movie in question. For instance, in "Lord, Love a Duck" Roddy McDowell played a high school senior when he was 36 (during shooting). In "The Sons of Katie Elder" John Wayne played the eldest of four sons at the age of 58, and looked it; the problem was the youngest son was 18 (in the story).

The bottom line is that producers don't care about actors' ages, generally speaking. They care about profits, which are linked to (1) star power and (2) the proficiency to pull off the role.

People complain that Wayne was too old for the part in "Katie Elder" but -- without him -- the film either would've never been made or wouldn't have been nearly as successful. Phoenix is arguably too old for "Joker" but, without him, it wouldn't likely be half as effective.

There have been a number of different interpretations of the Joker.
This one was as unique as the rest, while still staying the same character.

Think of Hamill's Joker!
Insane (theyre always insane), does song and dance, leaning toward Nicholson's portrayal... but he casually turned Gordon's niece into a cripple and THEN raped her, took pictures of the rape and did a song and dance about it while he tortured her uncle!!!!

Why would he get so dark?
As he would say, "we wouldn't get it".

He doesn't do things for the same reasons, or by rational, that we do.
That's why its so hard for Batman to figure him out.
He operates on his own insane wavelength.

There is no real Joker.

They are totally different.

Ledgers had a legit philosophy of life called Nihilism, which means life is about nothing and there's no point to doing anything. That is probably true and so his character wasn't insane and not the Joker.

This Joker sees the horror of life to be a crazy joke while at the same time hating people for making it a joke. That's very close to the real character.

Murray tried to play him, look what happened to him. Arthur might be dim, but as the Joker he is unpredictable.

Those are good points.

However, I think the man was childlike and seemed innocent because he was blocking his true personality. He was trying to be good and it made him seem naive.

If you notice, he got more confident as the film when on. He was more confrontational, less scared, not afraid of violence, etc. In the end, a kind of cult forms around him and I would assume in the future he feels fully empowered to let it all out.

For Batman purposes, I wish they would have shown that he was a closet genius somehow.