"Advice is what we ask for when we already know the answer but wish we didn't..." -- Erica Jong
As I understand it, the reasons to get a PhD don't have much to do with getting a promotion or changing jobs. It's more about following a passion, doing something for yourself, and apprenticing in an area of research. I'm sure it varies from person to person.
At any rate, I don't think I can give you an answer. At the company I work at we're always on the look out for folks with PhDs and I don't think we care about age. We do care a lot about other things though, such as expertise and presentation skills.
I hope that helps,
Jason