Petr
on your profile you have your interests listed as "extreme sports".
think that may have anything to do with it?

Not gonna judge..
I'm 32 years old. I've been working on my knees in some way or another since I was around 20 (Cement finishing, bricklayer helper, and now setting tile pretty regularily)
I don't really have any pain in my knees, but they are starting to make some strange sounds (like cracking knuckles) occasionaly

The only time I have any pain at all is when it's my own dumb-ass fault. "my knee pads are way over there in the truck, aww screw it It'll only be a few minutes"
My knees HATE kneeling on a cold hard surface. I know that if I do it even for a few minutes they will feel sore for a few hours afterward.
I take this as a sign to smarten the hell up. I think I'll be okay to continue on in this trade for many years, but thats going to depend on me. I know my knees are a weak point (although I've never had problems with them in my life) and I know I need to take precautions.
I think eventually they may degrade to the point that it starts to really slow me down, although I think that if I take care of myself I should be able to hold that off for some time.
As for "luck" I guess that probably plays a role for some. But that "luck" I think will eventually wear out. I havent heard of anyone who's knee's were harder or more durable than concrete!
Now when I look at my 21 year old helper...
I would say his interests could be classified as "extreme sports".
He's broken well... just about everything from the waist down? I donno. I know he broke both his ankles last year (was riding on the front of an ATV and crashed head on into another ATV) and was off work for almost a year. Now he's full of pins and whatever else they put in there.
Especially in the winter months I see him slow down a bit. My first reaction is "move your ass...."
But then I remeber his ankles hurt him a lot in the winter. At the age of 21 he already has to slow down when it's cold out. Whats that going to mean for him in another 10-20 years?
Not going to judge him either though.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
Holy S@#! !!!! What a ride!"
unknown author (as far as I know... i just found it on the internet

)