This should start a great discussion...
I'm older too (34) and share your concerns on this. Granted, I'm still a pre-med and once I (hopefully) get into medical school my interests/feelings may change.
I have always been on the fence about family medicine or pediatrics. My reasoning? Wait for it... Wait for it... I have no compassion for adults who do not take care of themselves. I would have an extremely hard time standing there silently if an adult patient was in front of me in the same scenario you presented above. On the other hand, I feel that infants and adolescents are completely innocent, since they don't have the mental ability to comprehend the results of the actions; diet, exercise, etc. (Of course the "mental ability" argument could be used for a lot of adults too I suppose.)
We can take it a step further in that many people become physicians so that they can solve complex problems, they couldn't care less who the patient is, they simply want to fix the problem. (ie. MD) No that wasn't a cheap shot, I'm simply referring to the philosophy between MD and DO. Part of the reason I look forward to becoming a physician is to solve medically-related problems, but just as much so to help people, spend time with them and be a contributing member of the community.
The question I'm dreading the most in interviews... "why do you want to be a doctor?" I of course have several of the usual answers, but the real reason, I think can't be expressed in words. I mean why does one like apple pie?
What motivates me as an older student? Lots of things. I love challenges. I've wanted to be a physician since I was 16. Mainly, getting up every day and feeling like what I'm doing is actually making a difference in the world/somebody's life.