I've been feeling lately like I'm middle-aged. It's not something I could explain it in any clear way, beyond the obvious things (like having a couple of kids, a job that I need to pay the bills, thinking about whether I'm saving enough for retirement, etc.). Besides, you can have those things and not feel middle-aged.
Then, tonight, it hit me: you're middle-aged when you read the vast majority of things because you need to read it, not because you want to read it.
Take Supercapitalism by Robert Reich. I'm not far into it (partly because I strongly suspect I'm not going to be joyful about what I learn), but if it's anywhere close to as good as The Work of Nations then it's going to be amazingly good. Now it's true that this sort of material does hold some interest for me. But not enough to represent something like 25% of the books I've read in the last year that weren't directly for my job (or reading to my kids).
So it's not the affair with the 'other woman' (haven't had one), nor the sleek sports car (don't own one) that defines middle age. It's when you spend practically all your time reading things you need to read and very little time reading things you'd like to read.