For me, it was just before 4th grade, to get into the advance classes in school. I tested at 135. And proceeded to be as lazy as possible until I graduated high school. I haven't done any critical thinking problems in years, haven't studied math at all since I was 17, and just took a test and scored 119.
Makes me feel even worse about being a slacker.
If you're interested, though, right now, you can take one for shits and giggles at IQtest.com
Never took one. Haven't really had a reason to I guess.
I used to work with a guy who was in Mensa. And, I hate to sound elitest, but when you work with a population of people who don't care for any intellectual pursuits at all, you tend to come off as holier than thou. So I would go about life and hear stupid shit and just say "Wow, I can't believe you just said that to me."
And then I'd talk to John, who has a degree in patical physics, and I could always tell he was a step or two ahead of me. He'd get this wry smile on his face, like he was teaching me from the Socratic Method and tilt his head back a little, and I'd sit there feeling like an idiot. (He wasn't intentionally doing it, John was, and always will be, the nicest person I ever met.) I would always wonder "Man, is that the look I get on my face when I talk to people who are 'stupid'?"