It also shows the Humans being slaughtered on mass in the hunt. But to be honest i first saw this film when i was about 8-9 as did everyone i know, this was back in the 70's and it didn't bother us at all. Kids were pretty tough back then, perhaps not so much today.

One of the scenes early in the movie shows that some humans had been strung up by their feet up.
Laugh while you can,monkey boy!

It did get a G rating, which doesn't mean much, as the MPAA at the time probably saw it was a sci-fi flick and just slapped a G rating on it. The film and subsequent franchise was a huge hit w/kids, eventually spawning a cartoon and a toy line from Mego, but I don't see any concessions to kids/families in the original film. It's got a reasonably sophisticated socio-political subtext, a dark and dystopic tone, plenty of violence and even strong language and some brief nudity. Would probably be at least PG today.
I first watched it at around 8 years old or so, and I'd say that's about right. Heck, it's tame compared to prime time tv here in good 'ol 2016.