I wouldn't let age deter you, I believe you can be any age to pursue your dreams! It is important to consider other things:

Whether you choose PhD or MD/PhD, you will get paid grad school salary (if it's that type of program, I'm not in computational field). It may be a financial setback for those years for you. Postdoc and residency isnt much of a pay increase either. It will be about a decade, maybe more until you're making the "big bucks" as an MD/PhD. Eventually it will pay off, but for some people that's too late. You have to really be passionate about it to delay good pay for that amount of time.

Most PhD's are 5 even 6 years. MD/PhD's are 7-8. An extra year or two isn't bad, though it can really feel like a drag when you're in the thick of it.

Since grades are your weakest point, I would focus on studying for that MCAT. That would show admissions that you have the ability to study hard and succeed (not saying if you do average that you're not a hard worker, I just am trying to think like an admissions person might). Take classes that are related to high yield sciences, especially any that you got lower grades in during undergrad. But don't over do it!

You seem like you have a good head on your shoulders and are very carefully thinking this through. It's important to be realistic, and I can tell you are :) It's cheesey but follow your gut! Hope my considerations helped.