The age at which people start feeling old is between 45 years old to 55 years old depending on the person and the persons current health at the time they turn 45 to 55 years old.
However if you're pretty healthy and stay active then you may not start feeling old until after 60 to 65 years old.
The healthier you are and the more active you are the less old you'll feel at certain ages.
I know an 80 year old lady who jogs nearly everyday and she says she doesn't feel old at all and she has taken good care of her health and skin and she looks maybe 60 years old but not 80.
So some people never feel old but others feel old sooner.
My father is 65 and he says he doesn't feel old and he can hike and climb mountains, climb stairs etc with no problem.
He also goes jogging and running and feels very good for his age.
I hope to feel very young and healthy at his age and beyond as well.