I first watched Back to the Future around June 2013, after years of people being appalled whenever I said I'd never seen it. It has since become my favourite movie, /by far/. There are some great messages about believing in yourself, having the motivation to make things happen for yourself, friendship, love, not giving in to peer pressure (in the sequels) -- "if you put your mind to it, you can accomplish anything." I love all of the characters (even Biff, who I love to HATE. And I may or may not have a crush on Marty McFly...), brought to life by some brilliant performances, most notably from Michael J. Fox and Christopher Lloyd who have the chemistry that make their friendship believable despite the age difference. I love all three movies in the trilogy, but this one is my favourite.
BUT MY GOODNESS, IT SHOULD NOT BE A PG. Several uses of "sh**", "damn", "ba***rd", a couple of "SOBs", and a few religious exclamations (I'm not offended by them, but others might be).
There's some flirting and kissing between Marty and his girlfriend Jennifer which isn't too bad; but they were planning a camping trip that he didn't tell his parents about, and he mentions that his mother would disapprove and he'd "get the standard lecture about she never did that stuff as a kid". I think older viewers will see what that might be implying, but this will probably go right over kids' heads -- so it's harmless enough. But then there's the whole mother-unknowingly-getting-a-crush-on-her-future-son thing, squeezing his leg, removing his pants while he's sleeping (we get a brief shot of Marty in his underwear), Biff attempting to rape Lorraine and his lecherous advances prior, Marty's somewhat iffy plan to pretend to "take advantage" of Lorraine in order to get his parents to fall in love (but he ultimately can't go through with it), George being a "peeping tom" (we see a woman (implied to be Lorraine) in her underwear from a distance).
The violence isn't really that terrible, apart from somebody getting gunned down, which isn't very explicit, and a couple of punches thrown. But some scenes may be frightening for younger viewers, like Doc getting shot, Marty almost fading from existence, the attempted rape. But all of these have positive outcomes and there is a happy ending.
There's some drinking, smoking (I approve of how Marty definitely doesn't like Lorraine doing this, which gives the message that smoking is bad), the Starlighters emerge from a car with lots of smoke billowing out, which is implied to be marijuana. But kids most likely won't know what "reefer addicts" means, so they'll think that it's just tobacco smoke.
All in all, I think that this movie is fine for children aged 12 and up. If Back to the Future had been released today, it would have landed a solid 12 rating (UK equivalent to PG-13) and NOT a PG. What were they thinking in the 80's?? But I absolutely ADORE the movie and the following two. I wish Back to the Future a happy 30th anniversary, and I shall be looking forward to the 21st October, 2015. If you have seen Part II you will know what this means. :)