I started installing/sanding hw in 1994 when I was 18 years old. I did the commercial gym floors for a few years then went residential, non-union ever since.
I always have worked for someone hourly(great wage) and running a business of my own whenever time allowed. Like many, I did it right and worked hard at a physical profession for almost 20 years. Recently I had to relocate to Chicago and all I do now is residential gyms.(Small boxes 700-900 sq, easy stuff outside of most being below grade and higher liability with moisture issues) My point to this story is that it's now physically tuff on me despite only being 39. My back gets tuckered out quick after a 15-20 minute hammer session, shoulders pop, wrists are weak,feet flat etc. I'll pound out 300ish on the average day after sub install and it feels like 1000, as what I could in prime years(GYMS). Its all I know, wood. It's how I make money but it is a struggle as I'm just no longer the same and it really sucks. Just wondering if there are any more out there that are/have gone thru the same situation. I used to be a beast, now just an old hound.

I only hope I haven't responded to this post too late . . . your "topics title" threw me a curve.
We are all going to reach that plateau, some just wont admit it. I'm there too. I knew it was just around the corner , so I made provisions. I slowed the pace & started working smarter. I've designed Floor Tools that are user friendly . . .much less fatigue ! Take a peek at some of the Ideas that have made being the Hardwood Floor Guy a little less fatiguing.

Ultimately, when You're looking at my photos, you'll probable realize that the Newest Idea (Licker-Tee-Split) . . .relieves the most stress!
search . . . (Dan R Braucht's photostream)

You can start getting more jobs of your own, and you hire younger guys who can knock it out. You might be working alongside them, but you're doing the less demanding parts of the job; you can still do racking, maybe. You can look into getting grants for going back to school in some other field. I know it's hard to match the money you can make at doing this, and age discrimination happens at 39-40, too. Some kind of change is needed--those are two options. Right now I still work with the crew, and I don't really want to be the Big Boss who just does estimates and shows up to drop off a box of staples. If I'm not going to train for something else, though, I've got to accept that I'll have to move into that role eventually. Maybe running the big machine only is something an older guy could do, but most companies prefer to have guys who can handle all of it.

There's always the inspection side of the business for income. And from what I understand there has always been a real lack of qualified guys in the business, or ones with hands on experience like yourself. I would expect this to be more so with your background and experience with site finished floors. It could be a game changer and opportunity!

http://www.nwfa.org/cp-inspector.aspx