What do movies about a boy wizard, a social-media phenomenon and a mind-bending world have in common?

Harry Potter and the Deathly Hallows,” “The Social Network” and “Inception” are all rated PG-13, which is probably all the proof any parent needs that not all PG-13 movies are created equal.

But that truth sometimes seems lost on time-starved, entertainment-hungry parents who inevitably think that their 11-year-old is so precocious that she or he can see all PG-13 flicks.

Several months ago a colleague announced that her soon-to-be-12-year-old daughter was going to a movie birthday party. The movie of choice for this group of tweens: the PG-13-rated “Inception.”

“That’s not a movie for 11- and 12-year-olds,” I offered. Never mind the violence, sexual situations and suicide theme. Its convoluted narrative would leave some PhDs scratching their heads. “They just wouldn’t get it.”

So my colleague called the parents having the party and invoked my title, saying “the KidsPost editor doesn’t really think it’s appropriate.” Another movie was chosen: the also-PG-13-rated “The Other Guys,” an incredibly raunchy, incredibly un-PC (and, to be fair, quite funny) Will Ferrell vehicle.

“I should have kept my mouth shut; they’d be better off seeing ‘Inception,’ ” I muttered.

It is, of course, easy to huffily wonder: What were the parents thinking? Honestly, they weren’t.

The mom saw PG-13, assumed it meant the movie was fine for all 13-year-olds and, because she was in a bind, decided to round down and assume that it was also okay for 11- and 12-year-olds.

The reality is that at least part of the blame lies with the rating system itself, which is not to say that PG-13 didn’t start out as a good idea. It was instituted by the Motion Picture Association of America in 1984 in response to parental outcry over movies including “Gremlins,” which some parents considered too intense for the PG ratings they received. It was intended to give parents guidance, advising “some material may be inappropriate for children under 13.”

In recent years, according to the MPAA, PG-13 has become the second most common rating awarded — behind R — creating an enormous umbrella including this summer’s “Harry Potter and the Deathly Hallows — Part 2” and last year’s “Easy A” (which features faked sex and a teacher-student relationship). PG-13 movies can contain nudity, profanity, sexual situations, references to drugs and alcohol and violence.

But here’s the problem: Culture has changed so much that no self-respecting 8-year-old would be caught dead seeing a G-rated movie, ditto for most 11-year-olds and PG-rated fare.

A Harvard University study from 2004 found “ratings creep” with more violence, sex and profanity in every rating than 10 years earlier.

Jane Horwitz, who writes The Post’s Family Filmgoer column, doesn’t mince words on the topic: “PG-13 has become largely useless. It implies that the movie is okay for middle-schoolers, and that’s the problem.”

So what’s her solution? “PG-15, which I’ve been proposing for years. It simply says, ‘this is okay for high-schoolers but not okay for middle-schoolers.’ ”

Horwitz also thinks that the only PG-13 movies that 11-year-olds should see are ones that their parents have seen first.

Or — and as a parent of teenagers and Jane’s former editor I feel uniquely qualified to say this — at least read what Jane has to say in Friday’s Weekend section or at www.washingtonpost.com/movies.

No ratings board will ever know your child as well as you do. Find out what’s in the movies your child wants to see. And be willing to say no sometimes. It’s your job.

Tracy Grant, the editor of KidsPost, writes about parenting issues every other week. Send ideas and rants to grantt@washpost.com.