Was that a young Bruce Wayne in the Joker trailer?
At one point, at around the 1:47 mark in the Joker trailer, Arthur is seen engaging with a young boy.
That’s why we can have a middle-aged man engaging with a very young Bruce Wayne, or else Joker would be getting social security checks by the time he was actually squaring off against Batman.
This young boy is played by Dante Pereira-Olson, who is credited as none other than Bruce Wayne, the man who goes on to become Batman.