After working 10 years in the non-profit sector, will getting a PhD open new doors? I am trying to come up with criteria to decide if I leave my job and start a PhD I was recently offered. One issue is the impact the PhD will have in my career: will it open new doors, will it make me a more attractive candidate to work for the government or the non-profit sector where I have been working for the past 10 years? Or should I just continue working on the issues I already have experience with and a network?

The main reason I am considering the offer is because I love the topic, I have been wondering about it for many many years and I would like to become an expert on it. It will be a big change for me, financially and geographically, as well as socially. PhD students can be very young these days.
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.