Being a Mountain Man wasn't a long time career job for the most part. Most didn't last all that long and for those that did they were pretty bunged up by 60 and out of work.
The "Old Mountain Man" is pretty much a myth. A few made it, but essentially it was middle age work.
Addition: And average middle age came quicker than today.
Addition 2: Thinking about it, for a variety of reasons nor was it work for the slow and creaky which could get you killed when moving quickly became important. Hobbling around the Mountains wasn't an asset.
