I will have to say, if you are unsure about doing a PhD, you shouldn't do a PhD. You have to be certain with what you want to do with a PhD before you start. It is going to be a significant sacrifice, and it really takes a lot of mental fortitude and requires emotional support to do a PhD.

From how you describe your situation, I would say, no, you shouldn't do a PhD. I can't see a good motivation for that. You don't have to worry too much about your GPA, it may cause some employers to skip your CV when you apply for jobs, but it is your ability to carry yourself during interviews that would be more of a determining factor of whether or not you are hired, and after you are hired, your GPA will never come up again, and from there it is your performance in your company that determines if you will a successful career.

Anecdotally, PhD drop-out rates is about 50%... (50% of PhD candidates never finish their PhDs) While that may seem high, but in that 4 years and looking around my lab, I would say that that number is about right.. Sometimes better opportunity comes around and they drop their program.. others may not feel that they are making sufficient progress after 1 year, and they decided to drop before investing the rest of that time... So unless you have a clear idea of what you want to do with that PhD, you should not do a PhD.

You should also know that in that 4 years, your fate lies fully in the hands of your supervisor.. I have heard supervisor horror stories and am really thankful my supervisor was determined to help me earn my PhD. So you need to know that your supervisor has your interest in mind before you start a PhD with him/her. Talk to your supervisor's other students and try to gauge if he/she is more interested in helping you or is more interested in using you as cheap grad labour.. If you don't trust or have sufficient faith in your supervisor, you should not do a PhD...

However, if you have a clear motivation as to why you want to have that PhD, if you have an inquisitive mind and enjoys dissecting problem, if you dream about problems at night, and wake up in the middle of the night to write down your thoughts, if you enjoy reading tonnes of journals ( and murdering trees.. ) (ya,ya i know i could have used a kindle... ), if you enjoy talking to others and learning from them, and nothing excites you more than to talk about some esoteric thing... if money is not the motivating factor (pay as a PhD grad are decent, depending on the field, but in that 4-5 years, you would have overtaken that pay if you were good enough to do a PhD) , and you can balance your living expenses from just your stipends and your pay as a TA,... If you can plan your time well and know that every hour you procrastinate is an hour extra you have to do in your final year...
first go see a therapist (something is clearly wrong with you), then go ahead and go enrol for a PhD programme...

My background: I worked with a major semiconductor MNC for 7 years before doing my grad studies. So I can definitely empathise with you when you say you are apprehensive because of your age.
Having said that, i don't think that age should be a limiting factors to consider doing a PhD. Family circumstance maybe.. (it is quite difficult to do a PhD if you have young kids you have to worry about and i have the utmost respect to my fellow PhD mates who had to balance between their family and their program) but many of the PhD students I know has both industrial experience and are well ahead in their age... and it does give you insights that the fresh out of Bachelors student might not see immediately...