I am sure my used TCR has more miles on it than that and I have no reservations about it at all. I had a 7 year old aluminum cannondale caad 3 frame that I rode the CRAP out of offroad every one of those years. I don't mean climbing and descending normal trails, but regular riding on really rough stuff like phoenix's National Trail. Over the years, my tires flicked up huge rocks that dented and bashed the downtube. It still rode and performed like a champ until it was stolen. Now it is probably parked behind some crackhead's apartment in the rain.

I realize that an mtb frame is somewhat sturdier than a road frame, but if that road frame has been used for its purpose (road riding), I would probably not shy away from it at all.