It doesn't get any more grown-up than 40. But I found that, with this realization, my shoulders noticeably dropped. I don't have to try so hard to be in the know. There's a freedom in becoming a bit of an old fart.
What would the world be like if even half of us refused to look at a calendar again? Or stopped counting the number of candles on our birthday cake? Or ceased looking in the mirror and obsessing about the number of wrinkles on our face or the size of our jeans?
We can easily end up spending the last half of our lives wrestling with the paradox of "How old I am isn't how old I feel!" I believe this internal conversation about "How old I am" versus "How old I feel" serves a purpose.