Age is not the issue, salary demands could be. Look at it this way, your salary demands would probably be higher at 29 than at 25. Unless you can prove you are really worthy, the 25 might be an easier hire. Keep in mind that you would've gone through ~5 years of lost income so you might overvalue yourself. You might also be a little arrogant (just because you have a PhD which in theory tells the world you are smart) and people might not like that. I will probably be wrong but you will be rubbing with other really smart people through your PhD studies and when you go to the street, everybody else that don't have a PhD might look below your "smart threshold line".
In summary, if you want to do a PhD, go for it but don't do it for the money/job in finance afterward. Also, don't be an asshole if you work on the street.