Who can watch PG 13 movies?

Anyone. There are no age restrictions for G, PG and PG-13 movies. Contrary to popular belief, you do not need a parent to see a PG-13 movie if you're under 13. PG-13 means parental guidance is suggested for kids under 13. That's "suggested," not "mandatory."

The only age restrictions are for R and NC-17 movies.

However, not all countries have the same rules and their equivalent classifications may be more proscriptive.