On TV it was probably a Hammer horror movie I stayed up late to watch. In the cinema, I remember lying about my age to get into The Good, The Bad and The Ugly. After that it was a whole batch of grown up films like Dirty Harry, French Connection, Shaft and The Wild Bunch. Got them all on DVD now, they don't make 'em like that anymore.
When I was legally old enough to see an X rated movie it was One Flew Over The Cuckoo's Nest.