I'm at the start of my career and decided to join a startup now. I'm at a point in my life where I don't need to support anyone other than myself, and can afford to work a lot for relatively low pay because it's fun to build something new from scratch. I can afford that this flops.
In ten years, I'm hopefully in a place where I want/need more stability, free time, predictability, and better pay. Either the startup works out, and I can have that, or I'm hoping that the experience I get working on it will allow me to get a "normal" job when I need it.
Edit: What's with people downvoting me for saying that I personally have joined a startup because I personally think it's fun and rewarding? Jesus...