I'm not someone who paid much attention to big birthdays before. My 18th? I was pretty much functioning as an adult already, or so I thought. My 21st? Not too interesting. 25? I was too depressed to care one way or another about a quarter century. 30? I had just started a new career and was too excited about life to be interested in a decade birthday one way or another.
Forty feels like a death sentence. Mostly, I think, this is because the great failures I've had in life could mostly only be fixed in youth. So 40 is not "old" in the sense that I'm about to drop dead or move into a nursing home, but it's become for me a symbol of missed opportunities and the finality of life. I have reached a point where I can see what things are going to improve bit by bit and which things I'm probably stuck with, and the latter is hard to swallow.
When you're 15 or so, the world informs you that you're going through a lot of changes, that life is going to be very unsteady, that you'll feel out of place and very alone, and that you'll feel a terrible sense of urgency to change things that you really can't. What they omit is that it continues for the rest of your life.
A month left of being 39, and for some reason I care a lot about that, in a bad way. It's a puzzle.