When I was watching the DVD, the person doing the commentary said that certain things in the movie plot was changed because the producers wanted it to be a family film. WTF? There is NO way I would let my young children (if I had any) see this!
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.
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.