DS1 and DS2 are both 14 (15 in two weeks) and I've pretty much let them watch anything up to a 15 since they were about 11, as they were both sensible kids and most of the time they were watching the films with us anyway.
18 rated films are kind of an approval basis. DS1 is a horror nut and I have no issue with him seeing things like The Exorcist or Friday the 13th. Anything like 50 Shades of Grey, they'd have to come to me first to have a discussion on why it's not a healthy relationship and all that, and if they understood it I'd probably let them watch.
Games don't come up that often but I've pretty much let them have GTA and CoD since they were 12 as me and DP are both gamers and understand what they're playing. We wouldn't let them have horror games until this year though because we're aware they affect you differently to horror films.
How firm are you on age rating things?