I have a PhD in machine learning and have had a fairly productive career now spanning almost thirty years. Even a few years ago, I used think that I would keep at it indefinitely. Lately however, I'm finding that the field simply just does not interest me as much as it used to. Contributing to this is the knowledge that I am financially independent - I just don't need to work at this point. I'm planning to quit later this year and spend more time on hobbies and studying cognitive science on my own.