I turned 30 my first semester at SGU. My roommate was 36. We had a smattering of guys in their 40s and one was in his early 50s. My roommate was a practicing lawyer and flunked out. The guy in his early 50s is now a practicing GP in Canada. I know one of the guys in his mid 40s could not pass his second USMLE step exam and dropped out about halfway through. I'm currently a practicing emergency physician in the Southeast US.
I guess what I'm trying to say is that it's more about the individual and not the age but it does get more difficult to discipline yourself to the rigors of academics the older you get. Contrary to my classmates, I found the first 2 years of intense book study to be much more difficult than the last 2 years of clinical rotations and "hospital work". Good luck.