For me it wasn't 35, it was around the time I became a "real adult" with a house, wife, and kids. And it wasn't that I began to hate my job...I hated having a job. Things that were previously accepted as just a fact of life became very annoying. I have to wake up, leave my kids to drive into town, then sit there 8 hours before driving back among a sea of other cars? Five days a week? I started to do the math and realized just how much TIME - which is obviously quite finite - I was spending either getting to work, being at work, or thinking about work.
Don't get me wrong - I'm glad to have a job that allows me to take care of my family, but man it sure put a bunch of stuff in perspective. Like cars. I could buy a new car...but then I'd be going to work to afford the car that gets me to...work. F that, I'll drive my beater until it dies.