I left a career to do a PhD at age 42. It definitely helped me take on challenges that require much more technical knowledge than if I had stayed on the "tinkering" path.

The distilled version of my lessons learned: 0) Hard to do a PhD part-time. Consider "retiring" for a while.

1) Your advisor will be focused on cranking out publications to advance their career. Not always the same as increasing the amount of knowledge in the world. You might be viewed as free labor in this effort. Protect yourself.

2) Your advisor will not help you manage your time or set goals. In fact they might need help in this area themselves.

3) If you haven't been in school lately, or deep into work related to your topic, expect to spend a lot of time "getting the rust out". Painful but useful.

4) The younger brain can solve problems more quickly, the older brain can pick better problems to solve. Your advisors won't know this either.

5) A storied career in academia is unlikely for an older student. (See Robin Milner for a great exception.) Keep your business network alive and growing the entire time you are in school.

6) The #1 goal is to finish. OK to repeat that to yourself over and over. Your thesis need not be the last word on a topic to be acceptable or even good.

7) You will feel like an idiot every day until you suddenly realize nobody can answer your questions anymore. That day you should start writing the answers in your thesis and finish it asap.

Finally, other than all the pain and anguish, it is an invigorating experience to find the brain still working.