When I was aged 21, 40 seemed terribly old. It seemed (because it literally was) a lifetime away. Now I'm forty - looking back - 21 seems like only yesterday. Time plays funny tricks on us.
My last significant birthday was 30. A lot has changed in the last ten years.
I have kids. :-) :-)
When I was 30 I still had thick lustrous black hair. It was a waist high meadow of waving long grass. Now it's a worn astroturf pitch. When the barber asks 'What can I do for you?' A raised eyebrow that says 'Nuke that mother' is all that's needed in reply. The nuclear option (#1 blade all over) is the only sensible option these days.
There's lots to feel good about. I don't feel old for one. I haven't turned into a miserabilist curmudgeon and have no plans to do so soon. Turning 40 feels good.