Yes, adult life is just a parade of minor disappointments — some of which come in the form of a prehistoric shark movie starring Jason Statham.

Imagine, if you will, a 12-year-old boy with a mullet. He’s living in Kansas, circa 1997, and he’s just purchased (at a grocery store) a book about a giant shark called “Meg” by Steve Alten. It will soon be the mulleted boy’s favorite book.

Shortly after, he will discover the Internet — in particular, the movie Internet, which reports the developments of upcoming film projects. It is reported that “Meg” will soon be a movie, and the boy likes movies, so this is excellent news.

Years pass. Decades.

The boy grows old, bitter. The “Meg” movie project sinks into development hell, plagued by false starts and bad luck. It goes to multiple studios (Disney, New Line) and through multiple filmmakers (Jan de Bont, Guillermo del Toro, Eli Roth). It tries to beat 1999’s “Deep Blue Sea” to theaters but fails. It gains traction in the early ’00s but stalls, due to 9/11 (audiences weren’t in the mood for disaster movies at that time).

But somehow — through Internet fan petitions and the tireless efforts of the author — Alten’s “Meg” now comes to theaters, 21 years later.

It’s called “The Meg.” And it sucks.

To be fair, “The Meg” is neither good enough nor bad enough to really care much about either way. But the movie’s been in the works so long it can now legally drink. One might have hoped the time was well spent. That the final product would be something more than a soggy late-summer mediocrity.

But alas, it is not. And the last bit of joy in the mulleted boy’s heart has finally died. Everything will let you down in the end. Even the giant shark movie.

Based loosely on Alten’s book and directed by Jon Turteltaub (“National Treasure”), “The Meg” stars Jason Statham as a deep-sea rescue diver (Statham, FYI, was once a competitive swimmer and diver) at an underwater research facility near the coast of China.

Funded by an obnoxious billionaire (Rainn Wilson), the facility’s purpose is to find the bottom of the ocean beneath the bottom of the ocean. You see, what we think of as the sea’s deepest depths is just a foggy layer of really cold water. Or something.

Anyways, the scientists go to the deeper deep and get attacked by a megalodon, a 90-foot-long shark thought to have been extinct for millions of years. The Meg follows them back to the greater ocean, and you know the rest.

Recipe for “The Meg”:

» Stir in an ensemble cast of expendable scientists and divers (played by Li Bingbing, Cliff Curtis, Ruby Rose, Robert Taylor).

» Put an annoyingly precocious child in peril.

» Add jokes. (They don’t have to be funny.)

» Toss in some dodgy CG effects (which at times look just marginally better than those of a certain series of shark/weather-themed SyFy Original movies).

And mama mia! (*Italian chef kisses fingertips*) We’ve got ourselves a subpar monster movie.

But if “The Meg” makes few efforts to be a good movie, it at least keeps things light and breezy. Alten’s book was a relatively serious sci-fi horror story, but the movie (rated PG-13) is leaning toward something goofier and more family-friendly. The tone often approaches outright comedy, even as the film delivers little in the way of actual humor.

But Statham’s Cro-Magnon charisma and the always-interesting Ruby Rose keep “The Meg” from sinking entirely. And the shark occasionally looks good.

Any new shark movie is inherently in a tough spot because the first notable shark movie (“Jaws,” obvs) not only ended up being the definitive example of its genre but also, you know, one of the greatest movies ever made.

A more realistic expectation for “The Meg” would have been: “As good as ‘Deep Blue Sea.’ ”

But it’s not even in the same league. “Deep Blue Sea” is great because it was at least kind of serious, which made it hilarious.

It had camp. It had Alzheimer’s sharks. It had LL Cool J bantering with a bird, and Samuel L. Jackson being chomped in half mid-monologue. Man, “Deep Blue Sea.” What a good movie.

The message is clear: Sometimes you wait your whole stupid life for that perfectly bad killer shark movie. But there it was, staring you in the face the whole time.