This is similar to my experience (mid 30s now). I remember sitting at my desk at work, staring out of the window, thinking "wow..so this is it". Would've been about 28-29. I think a combination of things got me to that headspace: end of the education-career pipeline, having bought and settled into a house, and having no kids. I reached a point where I was no longer occupied with achieving "my" goals, and I was left to figure out what my life was going to be. I guess suddenly becoming aware of that responsibility isn't very pleasant; before, I was just on the rails society lays out. School, job, marriage, house, kids, career, etc. Easy.

I'm not much interested in the pedantic debate taking place elsewhere in here about what truly qualifies as an x-life crisis. To me, it's a point in your life where there's some anxiety associated with the direction of your life. I'm more interested in how people came to think about their lives in and after those moments.