There is a current student at my school who was mid-thirties when accepted to a non-MSTP MD/PhD program. However, this person is the only one I have ever heard of who got accepted at that age, and I don't think it's very common. This might be in significant part because so few applicants to MD/PhD programs are above age 30. Even so, if you are older than 30, you should seriously consider having a backup plan, especially if your stats and research credentials aren't stellar (which this person's were). I'm not saying you shouldn't try at all, and I actually think that everyone should have a plan B when they apply to medical school anyway. However, when you're trying to blaze a relatively untrodden path, it's especially important to have that safety net there just in case you need it.
Ultimately, it's helpful but not essential to have a PhD to do research; there are many other ways to get the necessary research training for a physician scientist career besides going through a formal MD/PhD program. Besides MD-only, another route you might consider is the five year research-MD programs like Case Western's CCLCM and Pitt's CTSP/BSTP. These programs provide tuition support for the students, although not a stipend. If you end up doing MD-only, you can also look into doing a research year or PhD while in medical school, or even doing a research fellowship after you get out of medical school.
Best of luck. 🙂