Is the 18+ rating in the UK and wherever else that uses it the same as an "X rated" movie in the United States? Or is it "R rated"? I'm pretty sure you mean "R" (Restricted). In the USA, kids are allowed into R-rated movies only if they have a guardian with them.
Not really sure of the translation.
In the UK we have the following;U, PG, 12A, 12, 15, 18 and R18
U is Universal. Anyone can see it.
PG is Parental Guidance. Anyone can see it, parents recommended for younger kids.
12 is 12 years old minimum.
12 is 12 years old, or younger with an Adult. (only applies to Cinema, not purchasing films).
15 is 15 years minimum.
18 is 18 years minimum.
R18 is 18 years minimum, but it categorises special interest films and research. So you'll only find it in love-shops and researcho theatres.
You can't watch a 12, 15 or 18 if you're younger than that, even with an adult.