The Avengers aren't 60-ish years old.
The way i've always looked at it, people change over time. Luke changed the second he killed his father in RotJ. Luke was never going to be the same person he was in the Original Trilogy and I think to expect that was a pipe dream. Luke did try to train others to fight the Empire, but it all went wrong with Ben Solo. He screwed up bad and let his Sister and Brother-in-law down. Kylo turned out to be exactly what Luke was trying to fight against.
It's a very human story. As we make mistakes and as we age, we make choices based on those mistakes, etc. This is all how he turned into the person he is in the current movies. I think they've developed his character very well, tbh.