I've had the experience that... I was told when I was separating from the military, at 25b(I don't know why and I didn't ask...), to expect a colonoscopy. It didn't happen. I mentioned it to my GP a couple of years later, he said probably between 30 and 35... Now I'm in my 40s. It's perpetually a couple of years away.

I get the sense from this, and from other anecdotes in life, that preventative maintenance, of all flavors, is something we used to do, for a brief period of time, but that we don't really do anymore.