I have friends in their 70s and they act much younger than their age. I have friends in their 80s, and this is where they start slowing down. I have friends in their 90s, and I would consider that old age. I don't have any friends nearing 100 yet. I would consider that not old age, but "golden" age.
However, when I was young, I used to think 65 was old. Why? I remember we had been asked a question in a class, about what writer should we consider funding so they could do their writing. It was a multiple choice question, and it listed a young person in their 20s, another person in their 40s and another person 65. I chose the one in their 40s. At that time, I pictured the 65-year-old as someone over the hill and not having much time to do any work. Boy, was I wrong. I'm seeing this now with myself. One of the people I studied recently wrote around 8 novels from age 65 until his 80s.