Helen, the novel your memory is almost but not quite delivering is "I Will Fear No Evil" by Robert A. Heinlein.

Heinlein's obsession with extreme longevity made it a recurring theme of his oeuvre, fully expressed in his last mostly readable novel, "Time Enough For Love".

After yesterday's bout of gardening, followed by emergency removal of the parlour carpet (tomcat with possible urinary tract issue), my whole body is calling me elderly today, and it has me convinced.

I don't squat or kneel well, thanks to old injuries, so I weed in the bung-up-and-bilge-free position, bent from the waist. Consequently, I can still put my hands flat on the floor without bending my knees (much), but it's a bit hard on the hamstrings and lower back.