Well, the first thing is that 34 is definitely not too old to start a PhD program. In my field, work experience is common before beginning a PhD, so most students were between their mid 20s and mid 30s when they began the program. We had a woman in my cohort start when she was 34; she would be 40 now, but she hasn’t finished yet so I can’t speak to how she would fare on the job market. In a more traditional field like history you would definitely be one of the oldest if not the oldest student in your cohort; in a field like IR or public policy, you might find yourself in good company.
Really, it’s all about your own preferences. Can you get through the program at your age? And what I mean by that is not is it possible for you - of course it is, and there are things about your experience that will make it easier than if you were 22, I think (a more narrow focus on an area of inquiry; knowledge of how the working world works; the ability to multi-task; discipline). But more - do you want to? Do you want to live on $25,000 a year, in tiny apartments or shared accommodations, in your late 30s? Do you want to work ~60 hours a week studying and writing and passing exams and taking classes at this juncture of your life?
Now outcomes. What I know I’ve only learned second hand from reading the forums at the Chronicle of Higher Education (which might be a good place to also ask this question, as lots of seasoned professors spend time there). There seem to be mixed answers. Lots of academics say that it doesn’t matter when you begin, that if you do good work in a good program that you can get hired in academic positions afterwards. Lots of academics say the opposite - that, though illegal, age discrimination does happen in the academy, as departments want the potential to get a good 30-40+ years from a new hire. The basic conclusion I’ve drawn is that it’s uncertain, and that different professors will have different biases.
Time-wise, is it worth it? That depends on the person, but I would say that unless you want a job that requires a PhD at the end - and have a reasonably good chance of getting one - it’s not worth it.
Financially? Also depends. I started my PhD program at age 22, and my stipend was $32,000. I likely wouldn’t have made much more money than that had I gotten an entry-level job in my field (my major was psychology). Later in my program I was able to take on additional jobs to tip my total salary up, so that in those years I was making anywhere between $35-45K depending on how many jobs I had. I’m currently a postdoc and I think that my postdoc salary is maybe slightly lower than it would’ve been had I earned a master’s with the work experience I have - but in 2016 or 2017 I’ll be making much more, potentially nearly twice as much as I make now. I borrowed some money for my PhD (mostly transitional living expenses) but far less than I would’ve borrowed for a master’s. So combined with the fact that the PhD enables me to do jobs I really want, I think it was financially worth it.
Would it have been worth it to me if I was already working, content in my field, making - let’s say - around $50-60K a year? Eh. I doubt it, unless I had a burning passion to be a professor or sensed that I couldn’t go any further without a PhD.
It is definitely not worth it without full funding, so don’t consider that, IMO.
Career-wise? That depends on what you decide to get the PhD in and what you want to do. If you want to be a professor (despite knowing that the odds are against you) then you have to have a PhD, so it will be worth it in opening those positions to you. I chose a field in which I knew there were academic and non-academic options, and I think it’s worth it for me career wise.
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The short version of my advice is if you have doubts, I don’t think it’s worth the PhD. And if you could be quite happy doing work that requires a master’s only, then do that instead. I think the PhD should only be pursued if you just have a burning passion to be a scholar and researcher in a specific field and nothing else will satisfy you - or, less dramatically, if you are okay with the idea of spending 5-7 years in school making $25-30K and answering (a) research question(s) that is/are important to you, with the knowledge that you may not use your PhD after you finish anyway - just to have the chance at maybe having an academic or research-related job. I fell into the latter camp because, as I said, that was going to be about what I would make anyway; so why not earn a PhD along the way?
I cant realistically take 5-7 years from my working life and expect to achieve all that by 40.