Getting Old?????

Yesterday was my birthday, and I am amazed to find myself at 58! When I was a teen, I thought that 30 was "adult", and "40" middle-aged, and "sixty" was pretty old. Well, when I hit 30-40-50.... I kept redefining my definitions.Its not that I think "old" is bad, but I don't FEEL old. In fact, as a result of my walking and weight loss, I feel younger than ever (almost... I am ignoring here my bad knees). OK. To be honest, I realize that I do not have the stamina I had at 18. But I have a lot more than I had at 45!
[Actually, a good question for discussion would be: “What is feeling ‘old’?” Our “love-youth society have perhaps led us to believe that growing old is not a positive thing, while in many ways it may be!] And in a way, I DO feel "old", in that I feel I have a vision and maturity and wisdom that most young adults have yet to gather.
But at almost-60 I am not willing to consider myself "old", rather "middle-aged" (even though some voice from within is whispering in my ear that I have undoubtedly passed the midpoint of my life span.......). I wonder what I will feel like at seventy????
And I look at the older people in my life who are WAY fitter than I am, and realize that the only real problem is not years, but how you feel!