In the UK, film certificates are a legal limit - regardless of the parents' wishes a child under 15 cannot get into a film rated '15', and a child under 18 cannot get into a film rated '18' (in theory, anyway). We recently had our '12' certificate changed to '12A' so that parents could give permission to their under-age kids. (The film that caused that shift was 'Spiderman'.) Oh, and films are never shown in an 'Unrated' version in cinemas.
Likewise, all DVDs are rated using the same system. (Occasionally, a film's rating on DVD will be different from the rating in the cinema, as was the case with "Starship Troopers". For a while, there was also a rule that two different DVD versions of the same film had to have the same rating, but that rule might have been relaxed. There are no 'Unrated' DVDs.)
Most films that are PG-13 over in the US are rated 12A here, and most films that are rated R seem to be rated 15 here. The 18 rating seems to get a lot less use here than it used to. (However, our rating body generally offer studios a choice - make some cuts for a 15, or accept the 18. Most studios make the cuts, which annoys me a bit.)
All of which is a long way of saying we don't really have this particular problem to deal with. Giggling teens and annoying mobile phones, on the other hand...