I read some research studies some years ago for my last degree level qualification, one in particular I passed onto my parents and my father actually passed it on to others. Can't recall the detail anymore but the gist was that people in a care home with average age of something like 80 were given ability-appropriate strength training exercises to do. After a number of weeks all had become measurably 'fitter' and some were able to discard their walking aids.
Use it or lose it in black and white: we can't all run marathons at 100 but almost all of us can avoid some of the supposedly 'age related' decline in fitness and ability. Another study I read of suggested that regular Tai Chi halved
the risk of falls in the over 50s or over 60s (forget which) - falls of course being linked to hip fractures. Some who fracture their hip never walk independently again so if you can avoid falling in the first place ....
It's ridiculous to describe 50 as old, that doesn't divide the expected lifespan up in any sort of meaningful or logical way. Some women still have young children at that age, occasionally someone falls pregnant then, men can continue to have children indefinitely so the human body doesn't seem to think 50 is 'old'. Arguably the body thinks our youth, defined as child bearing years, are done with at ('normal' not premature) menopause so you could say women enter middle age at that point.
Some decades ago 40 was considered the start of middle age, firstly the envelope has clearly been pushed there and secondly middle age doesn't last a mere ten years, we don't make 'old' people work full time for another ten or fifteen years up to 60 or 65. That is the age we can draw the state pension (AKA 'old age' pension) in the UK but even that is set to rise to 70 in recognition of our improving health wellbeing and longevity.