It depend on 'what you are' or 'think you are.' I'm seventy-five this month but quite a few people think of me as being around sixty. That's the difference between being 'young old' and 'middle-aged old.' 'Old-old' is ninety plus.
Years ago my then teenage daugher said to me: Dad, how old do you have to be to be middle aged?" At that time I said: "Do you mean young middle age, or old middle age? I was then forty-two. We are, afterall, middle aged for a least a couple of decades. This is all very nebulous stuff.
Then again, I was once informed that this is my 442 nd life as a human being, so how old would that make me! Quite frankly, I think we're all immortal and therefore ageless...However, if we're talking about human bodies...Let Nature tell you how old you are.