2001: A Space Odyssey can be shocking at times, but these shocks are doled out by implication rather than the visceral images that ping the MPAA rating system. There are a few "damns" and "hells" thrown around here or there, but you'll hear coarser language in a church quilters group.
Granted, there are a few murders. They're carried out in a bloodless fashion, yet they can still be crazy terrifying because of the greater implications of the destructive power of technology. Kubrick found a way to make the slow ticking down of vital signs on a computer screen far more unnerving than the fountains of blood provided by any horror flick. Freddy Krueger wishes he had half Kubrick's flair.
So it's a G rating on the MPAA's scale but still not kiddie stuff. Heck, even adults can't grok it.