Is it possible to start a PhD at 36 without taking a huge hit financially? I'm currently working as a Senior Engineer (Mechanical) for a large company. However, I am increasingly finding the work to be dull and lacking in challenge. I don't think I can bear to spend the next 20-30 years like this - I feel that I am not making use of my true abilities and I yearn to be doing something more novel and breaking new ground.

I live in the US (Boston area) and I have been looking around at what other opportunities are available; however, it seems that many of the more interesting-looking positions (which are more research-based) require a PhD. I majored in fluid dynamics and have some experience in CFD, so I am looking at positions that lean towards that area.

So, I am considering whether I should do a PhD, which seems to be a logical choice if I want to be doing more intellectually-challenging work. However, right now I am earning a good salary; I have a big mortgage to pay and a 3 year old kid to support. Doing a PhD seems like it would be a very difficult path to take, from a financial standpoint: not only would I have to give up my healthy salary, but I would also most likely have to pay a lot in tuition fees, in which case I would go from earning ++$ to -$, which would be very difficult to manage.

So, my question is: is it possible to do a PhD at my stage in life, whilst also paying the mortgage and supporting my family? What is the best way to balance this?

I should probably also mention that I have a Masters Degree in Aerospace Engineering from a top UK university (I moved to the US from the UK a few years ago).
I did my PhD in computer science in the Boston area. Through an RA and a bit of TA, I had my tuition, health insurance, and conference travel expenses covered, plus a stipend which has since risen to $36,800 before tax per year. (With a bit of googling you can look up what financial support various PhD programs offer, as well as health insurance costs for dependents -- for a large family, this might be $10,000.) That was enough for a modest but comfortable single person's lifestyle in expensive Boston, but I would struggle to support a child or mortgage with that. PhD stipends will vary by field (i.e., how much funding professors have) and by university (i.e., local cost of living).

You will know your financial situation and it's up to you to judge whether you can cut your expenses and/or dip into your savings to make that work. PhDs are getting longer, so keep in mind that you probably need to do this for about six years.

There are a few other options to make it better financially:

Ideally you can work something out with an employer where they will pay you while you do your PhD, probably in exchange for some part time work or a promise that you will continue working for them afterwards. Such an arrangement is more common for shorter programs like an MBA. 

It's also possible to spend some time during the PhD working in a company that is somehow related to your studies. For example, a lot of computer science PhD students will do internships at companies like Google or Microsoft. A 10-week summer internship could give your annual income a >$10k boost.

If it all gets too much financially, you can take a year or two off your PhD to work and then come back to finish. That's not ideal, because it interrupts your work. However, it's better than needing to drop out entirely.

Overall, however, I think it's unavoidable that a PhD will entail a financial hit. A PhD is almost never a good financial decision, but if you are passionate about research it's definitely worth it.