Well I'm 59 and can still work circles around any of the 20-30 year old techs in our shop. Granted they don't want to work hard and a 40 hour check is OK with them. The younger guys have very little upper body strength. I can still crank out 60-70 flat rate hours in a week and that can only happen if you hustle on each job.
I like to work hard but when I get home I crash into my recliner and take an hour nap and wake up for dinner watch TV and in bed by 11 o'clock.

Gus