Is 70 the threshold of becoming old?

Age-old question about getting old; at what age do we cross that threshold between being still kind of young and becoming an old person?

For years, I've thought that milestone was at 70, but why should it be that convenient round number and not 68, 72 or even more?

Maybe it's simply because I'm getting close to that number that I'm increasingly paying attention to it or that a good friend of mine recently “made the grade”, but, deep inside, and barring more arthritis and less brute force, I feel just, and function almost, like when I was 25, so I forget about these arbitrary boundaries and let's use a “floating” one instead!