I think the decline often *looks* sudden because the loss of reserve was gradual.
For years, your body compensates well enough to make you "feel normal". Then an infection, injury, surgery, spending too much time on getbig.com or period of inactivity pushes you past a threshold. Recovery takes longer, you lose strength, and that can become a shitty cycle.
There’s no magic age where this happens. Genetics, training, sleep, shitty wife and medical history all affect how much reserve you have. Key is the reserve (And no Ron Avdian, can't replenish reserves on Orange Chicken alone)
Staying "fit-ish" helps, but it doesn’t make anyone immune. If someone declines dramatically over weeks or months, I wouldn’t simply write it off as “getting old.”
I’d want to know what changed. Then treat it, rebound and get back to baseline.