When you start to no longer be any fun! Middle age is not a number, it is a state of mind, just like age. The concept of middle age and old are designed to denegrate people. Remain young as long as your body and mind will let you. Happiness is just a state of mind. If things go right, you can remain young a long time. It is others that try and put names on you. If you can, go for it!
Hey, you have to embrace your age. You're as young as you feel. One of my students recently told me that I was cool, because even though I'm really, really old, I still know how to have fun. And it's true (I'm 50).
I know, I do act pretty young for my age, my mental age is still in the teens I think. lol Just don't want to think of myself as technically 'middle aged', sounds horrible!
So I'm not even middle aged yet.........SWEET!!!!!!
Many don't make it that far, so I guess that we're all lucky! Shouldn't waste time worrying about anything else!