I think the censors have always reviewed what is and isn't suitable for under-18s as time has progressed.
Films that used to be classified as 18 (or "X" as it was then) are often quite suitable for younger people to watch (although not always, of course).
Film makers (and producers, especially) are keen to keep movies in the 12A or 15 bracket because they'll get more box office takings (this is especially true in America, where the dreaded NC-17 certificate is the kiss of death for many movies), which often means slightly toning down scenes of a sexual, frightening, violent or gory nature.
Monster movies have been downgraded on the whole; it seems that scary movies are OK for teenagers to watch - there's a film being advertised on TV at the moment which they're calling "the new Alien" which has got a 12A certificate, for example.
I'm OK with that, really - kiRAB have been playing console and computer games like Silent Hill or Resident Evil for years and I think they're pretty clued-up about scary monsters etc. The worst it's going to do is give them a few nightmares (which I've had as an adult after watching some movies), so what?
The line is still drawn at things like drug abuse, excessive violence (Saw, Creep etc.) and strong sexual content, which is as it should be.