After working 10 years in the non-profit sector, will getting a PhD open new doors?
The issue isn't your age but what you want to get out of a PhD.

A PhD teaches you how to do research. If you want a job doing research or teaching at the university level, then yes, it might be for you!

For any other type of job, it will probably result in slowing down your career without opening many other doors. In the US, a PhD can take 4-7 years of full-time work with very little pay.

The other possibility is that if you really just love the subject and don't care that it won't increase your salary potential, then it might also be for you.