From a financial and effort perspective, a doctorate is a terrible return on investment at this age. Back of the napkin numbers, couple years to get research part of CV up to snuff and apply and get in somewhere, 5 years of grad school, 1 year internship, 1-2 year research postdoc. So, if everything goes right, you have 9-10 years when you are on the job market, and most of those years are at a reduced salary, at least 5 of those greatly reduced. So, now you're 63/64 at the earliest looking for academia jobs. Call it ageism if you want, but academic departments are looking for someone who will be building a research lab and sticking around for a while, not someone who is already at retirement age. Now, if you just want to adjunct teach, slightly different story, but then you are relegating yourself to terrible salary.