I was 51 when I started my PhD in software engineering. Some thought it was a sort of mid-life crisis. Over the decades of working in various areas of IT I had a burning desire to research a specific area that held an enduring interest for me.
I need to correct your use of the word "study". Doing a PhD is about extending an existing body of knowledge in a small but significant way. So you need to already have a Masters level of knowledge in your field and to be reasonably up to date in the field. Once you start your research you survey your field, aka literature review. Then you formulate how you are going to contribute and get stuck into experiment after failed experiment.
I was most fortunate to have advisors from several other leading universities around the world, people who were widely cited in my field. My principal advisor was helpful in navigating the bureaucracy, getting funding for conferences and motivating me to write and present several papers at conferences in my field.
Doing a PhD is an emotional roller coaster ride. I suggest that you Google extensively to read about other people's experiences. Be warned, you will find a lot of humanities PhDs experiences. Ignore them. Science and engineering PhDs are very different so focus on those. I can recommend reading phdcomics.com - lighthearted, and very accurate. Writing the dissertation can be challenging. Graduating feels like crossing the finish line after running a marathon -- total exhilaration and you forget the character building challenges that you faced up to in order to get there.
