The med school population tends to be somewhat bimodal, with the "right-out-of-undergrad" to "two years research after graduation" crowd forming one curve and the other, older crowd coming from advanced degrees, other careers, black holes, what have you. I entered med school at 28 and was pretty much at the front of the latter curve, which had a long tail of outliers in their mid-30s and even early 40s. As I tended to associate with my own (more mature) crowd, I actually felt relatively young most of the time, and after the first 2 years of medical school I didn't give my age any thought whatsoever.
Feeling complacent is not a great reason for getting your MD, however. On the other hand, I'm not sure there are any good reasons, despite what we spout off on interviews. I honestly believe that medicine will always be a leap of faith, not only because you'll never know what it really means until you're knee deep in it, but because you'll be a different person with a different perspective on life and your goals by the time you've achieved it. That's not to say you won't be happy at the end (I am), just that it is near impossible to predict. Most people will find some aspect of medicine they enjoy, but most wouldn't have guessed what that would be at the beginning. A sad few will not find anything redeemable in medicine and either quit, burn out, or both.
