Depends on your personality and the jobs you've taken. I know auto mechanics wrenching it into their late 60s. A friend's dad owns grocery stores and still does stocking in his late 60s. He puts our step count to shame. I also know fly-by-night moving companies that work kids into the ground by having them lift things in all sorts of bad positions. I know impatient people who will lift with bad form because they're in a rush. (I don't want to imply that these are negative personality traits. Many, many people have a hard time dealing with the patience required to lift things properly for a job.)