28 isn't too old to start a PhD. Some don't start until their 50s or whatever, so it's entirely up to you. If you have a good research group then now is a good time to start. I'm currently 28 myself, although I dearly pray that I'm almost done with mine...
With that said, there are a lot of things to consider when deciding if you want a PhD. I entered the PhD program under a very bad mindset that I wish I'd had set straight as I started. To me, a PhD was just the 'top degree' that I could get, so why not? I had absolutely no perspective that a PhD is really just a certification that you can do independent academic research and doesn't directly translate to a 'top degree' directly. This, I think, would explain why I enjoyed my Master's of Engineering degree much more than my time in the PhD program so far. It really was a continuation of undergraduate engineering work, whereas the PhD was not.
The Bad
A PhD can take a very long time to finish. Some unfortunate souls may take 8 years to finish. I'm currently on year 5, although wrapping it up I hope. If this project falls apart I'm quitting.
Along with the amount of time it takes to finish, a PhD requires a veritable mountain of patience and self discipline. Many students I believe, including myself, finish their classes quickly and think they're almost done. This is not correct. The research part, which is core to what a PhD is, can be described as soul crushing at times. It is for me at least. You can spend an entire work week hammering away at a problem to make no progress, or worse yet, discover that the idea you had simply won't work. I've had 2 entire projects, with half a year or more spent on each, thrown away entirely because we discovered that they just weren't practical. Oh well. Start over. Better luck next time.
There have been periods for weeks at a time that I only ever left to go home under considerable amounts of anger, frustration and depression. I suspect that I'm not the only one that has felt this way.
I'm lucky that my current project turned out to actually be feasible. Even still we're having to cherry pick how to apply it to get a good enough result out of it to publish it. We've submitted a paper to a journal on it, but I'm 99% sure it will be rejected because of the results being weak. That means it will be somewhat harder to convince my advising committee to pass me and let me out of this place, but I have gotten to the point that I really do not care at all if my results get published. Supposedly you really need 3-4 or more publications during your time as a PhD student, especially if you want to work in academia later. I have 1 unrelated to my current project and I don't care. I really don't care at all anymore. I just want to leave and not throw away years of work - that is, I just want the degree so I can quit. I am legitimately sick of it.
Publications are an entirely new category of things I disliked about the experience. Once you've spent all of the time working on a project, you have to write up 10 pages about it, submit it to a conference / journal, then wait 3 months to find out that it was rejected. Start over and hope for better luck next time. Or, if you do by some miracle get accepted, you have to travel to the conference and present your work to a lot of people who probably don't really care about it. Oh, and I hope your department has a travel budget, else you're looking at $1,500 or so for a typical conference visit. Can't attend? They won't publish your work.
The Good
If you really like working on open ended problems, then it might be what you want after all. I found out that I don't. I really like concrete problems to be given to me with a clear solution for me to work toward. Some people like research for its own sake though. When I do have good results during research I find it quite pleasurable. I've heard a few people say that PhDs are good for your soul, and in the long run maybe that's true.
Maybe the prestige of being a published author appeals to you? I'm sure it does for a lot of people. It does feel nice to be able to say, "Yeah, I advanced the knowledge of humanity on projects X, Y and Z, and you can find my work in the following journals and conference proceedings..."
Or maybe you'll legitimately like conferences and getting to know people in the research field. My advisor talks about people he meets all of the time at conferences, and he seems to enjoy specific ones anyway.
It can be a source of income too, if you're lucky. If you have a good research group and an advisor with funding, you might be able to get an assistantship. That means that not only is the PhD largely free, you're getting paid to get it.
Other Considerations
I've heard many people say that a PhD is only bad for your wallet in the long run since it potentially robs you of years of experience and career building, leaving you with an advanced degree and little to no work experience to show for it. Furthermore, a PhD does not guarantee you a high paying job. I've known some that ended up teaching high school and I don't think they make much more than those with just an undergraduate degree.
Summary
I'm really not trying to scare you off. PhDs are important things for the world at large, and if you like research then maybe they're for you. I just found out quite late that I really don't like research and as a result the experience for me has been pretty miserable.
I'd strongly suggest talking to your advisor or someone in the graduate department to find out more. They'll probably just say I'm a sourpuss and that it's not nearly so bad, and maybe for a lot of students it isn't. A lot of the troubles I had stemmed from working an unrelated job while trying to complete my PhD, leading to not having enough time for either, really...