I did one in my later 20s. It took five year full-time. I was lucky to have some independent financing. I passed it but it didn’t set the academic world alight!
It has to be something you’re passionate about. I did an undergrad degree. In the last two years we had to choose specialist subjects. I chose one that seemed interesting. For one of them the students had to choose a topic from a list and give a presentation on it. I noticed there was a topic with only one book and a few articles on the reading list, so close to do that one, to be honest I think I was being lazy. I mentioned to the lecturer that there wasn’t much on the reading list for it, and he said that’s because not much had been written on the subject. So that is what set me off to do the PhD on it. Of course once you start digging you find more has been written on the topic than you initially thought, with lots of little articles here and there.
