I am 70 and still have the bike my father bought me when I was 14. I ride it every day. The only original parts are the frame, the rear brake post, and the head bearing nut.
I enjoy working on my bikes as they're fairly standard as far as parts go. Swapped the original 26" tires for 640Bs, removed front derailleur for more rear gears, changed hand bars, etc. The bike has grown and changed along with me through the years.