Back in the day, we did not celebrate adult birthdays like we do today. When my parents were, 50, 60, 70, 80 nothing special was made of it.
Whereas I did not celebrate birthdays in my younger adult days, I did make myself a big 50th. During the previous year I had survived a serious illness and become an orphan.
I most definitely did not feel, in any, old or decrepit.
Then years later my children made me a large surprise party which was great. I was then entitled to retire (although did not do so for several more years), and also got my bus pass.
I made myself a lovely big 70th celebration, was noticing loss of muscle strength, but was still feeling very little different than I had for the past twenty or so years,
My 80th, towards the end of the Pandemic was a hit or miss affair, although I did get for that what I most wished which was a get-together of my five remaining adult children, their partners and my eight grandchildren. The anxiety of recent times there and deaths of friends and relatives did have me feeling more vulnerable - and YES, I felt old now.
I would say, that since my eightieth birthday, there has most definitely been a downturn in my health and my everyday living abilities. Nearly four years later, I still live totally independently, but find I have much more trouble doing everyday tasks. Have given up driving, need much more sleep each 24 hours, do things much slower.
Feel that my mental capacities are pretty much unchanged, but very large changes in my physical ones, and talking to others in their 80's most say much the same thing.
So, I would say upto 70 is middle aged, 70 plus is elderly, 80 is old and 90 is very old.