Go for it, apply for a PhD if that's what you want, don't worry about your age. I also started my PhD later than most of the people in my department, and I really love to reply to my colleagues when they say: "I can't wait to finish my PhD and get a real job" with "I had a real job and I can't wait for the rest of my PhD"
Hope that helps.

Can't you just talk to your potential supervisor about your doubts? He sould be more helpful than anybody else on this forum or outside, I guess.
Honesty is the best policy, so they say...

I think you should read some of the professor's papers, find out what kind of research is done in his group, department etc., find a topic you might be interested in and then write a simple and direct email saying you're interested in that particular topic and asking him if there might be an opportunity for you to start a PhD project under his supervision. There's no need to write a fancy email at this point.
I hope this answers to your question, it was rather general...

In theory, I think you need to be VERY sure you want to start a PhD, because once you start it, you should to be motivated enough to complete it successfully. If you're not motivated enough, it will probably be a nightmare.
But
if you're very ambitious and...a bit masochist, I'd say: "do it, start the PhD, finish it and get on with your life" even if you're not sure or you think you're not sure.