About age

I was telling my father once about a man who had just died. "How old was he?", my dad asked. "He was old," I answered, "in his mid sixties". "And how would that be old?" he said, with surprise, or maybe with fear, I couldn't tell. My dad is 61 years old. How wise, you might wonder, but that made me think how relative the perception of age is. My younger sister once told me about a woman who is single and depressed. "How old is she?", I asked. "She is old," my 24 year old sister answers, "she is 29." I am not age-phobic but I also had that same weird look that my father had and I asked myself: when do we stop growing and start aging? Even my grandfather who is more than 80 years old, once said "poor guy, he died too early. He is only 85". Is it death? denial? or is it that people just feel eternal? What is the right age to say that someone is old? I do understand that it could be relative all throughout life. But then give me a number, 90? a 100?