I was half watching that new Joker film that’s just come on Sky recently.
It probably did explain it in the film and I missed it as my flat mate came home and started talking to me for ages, but I’m sure I saw a young Batman talking to a 50 year old Joker at the gates of Batman’s dads house. Weren’t Batman and the Joker both around 35 years old when they met in the Dark Knight?