Well for American (movie) ratings its usually 5 categories:
G - All Audiences
PG - Parental Guidance (but probably watchable for kids)
PG-13 (Some material inappropriate for kids under 13)
R - (Anyone under 17 has to go with an Adult)
NC-17- (No one under 17 can see it)
I just remember seeing plenty of PG-13 films that had themes like @vothnthorvaldson mentioned. As long as the actions weren't really being show, it seemed to get by. But I don't recall seeing drug/alcohol use and nudity in PG films but it's been a while since I've watched anything under PG-13.