That sounds like such a weird question!  The definition of “old” varies over time, culture, and situation.  It may depend on the age of the person defining the word. At 20-years-old, you may be called “old” by a 6-year-old.  A 20-year-old may think of a 40-year-old as past their prime. A 60-year-old may see a 70-year-old as elderly.  

Some people think of old age according to one’s chronological (numerical) age thinking that once folks reach retirement age, they are old.  In that same manner, folks who are under 65 are still very young in some eyes. Others may determine old age by one’s functional age, their physiological age.  More Many older adults may be in their 80s, but have the physiological abilities of someone decades younger.  

For those of us who are young at heart, we may come to know age as nothing but a number.  Yes, there may be some challenges as we live in our bodies longer and use them more, but I like to think that what makes us old or young is a matter of perspective.  Frank Sinatra sang it beautifully in his song “Young at Heart” (https://www.youtube.com/watch?v=61HW8uxzEZs).

“Fairy tales can come true, it can happen to you
If you’re young at heart
For it’s hard, you will find, to be narrow of mind
If you’re young at heart

You can go to extremes with impossible schemes
You can laugh when your dreams fall apart at the seams
And life gets more exciting with each passing day
And love is either in your heart, or on its way

Don’t you know that it’s worth every treasure on earth
To be young at heart
For as rich as you are, it’s much better by far
To be young at heart

And if you should survive to 105
Look at all you’ll derive out of being alive
And here is the best part, you have a head start
If you are among the very young at heart.”

* Inspired lovingly by my 94-year-old Hānai Mom.