It’s ironic that “Old” concerns the speeding up of time; the movie is so painfully bad that — in contrast to the characters, whose lives go rushing past — its 108 minutes feel more like 108 hours. Twice I had to convince myself not to simply get up and leave.

Sadly, “Old” comes from writer-director M. Night Shyamalan, whose 21-year career got off to a roaring start with “The Sixth Sense” and “Unbreakable,” fumbled through flops like “Lady in the Water,” “After Earth” and “The Last Airbender,” then recently recovered with “The Visit,” “Split” and “Glass.”

“Old” is what’s called a high-concept movie, where a simple central plot-hook dominates the proceedings — something like “Jaws” (shark at the beach!) or “Speed” (“Die Hard” on a bus!). The thing is, though, this key premise has to be rock-solid — and the one in “Old” isn’t.

The film is about a secluded beach where vacationers inexplicably begin aging super fast — like, five years older every couple of hours; they also find they cannot leave.

This whole idea is so patently absurd that it would need exceptionally careful plotting, acting and dialogue to pull it off; but “Old” doesn’t have a lot of that, either.

Among other things, you’d need more than the film’s three sets of actors to show kids growing up so quickly; and at 3500 calories per pound, where on earth would they get the food to put on so much mass?

Granted, several performances are good; but both Abbey Lee and the usually reliable Rufus Sewell choke on the scenery they’re chewing — with this painful execution further hampered by lame and laughable lines from the script.

“Old” then flies completely off the rails when two kids go from building sand castles to having sex, resulting in an instant pregnancy and arrival of baby, which then dies — all in the space of about 10 minutes.

There are too many other deaths, which often feel like lives being thrown away in typical horror-movie fashion; but I will say that Shyamalan — master of the unexpected ending — almost redeems the story in his final act. This doesn’t exactly make sense of everything, but it is at least compelling.

Yet even then, the conclusion overstays its welcome, clarifying items that really didn’t need elaboration.

The best you can say for “Old” is that it is at least ambitious; after sitting through endless previews so uncreative and redundant that I actually got out my book and started reading, it’s nice to know that Hollywood can still find room for an original idea.

But if all you have is an idea — that can, shall we say, get old really fast.