“Andrew Jenks, Room 335,” a moving and provocative new documentary showing on Cinemax tonight, begins as a whimsy when a 19-year-old decides to spend a month in an upscale assisted-living facility in Florida. But any hope that Mr. Jenks, the film’s precocious director and star, will turn out to be a waxen-fleshed, computer-age decadent prematurely bidding adieu to life’s bliss is quickly dashed.

Tall, dark, relatively handsome, and sloppily dressed (shorts, sweatshirts, sneakers, etc.), Mr. Jenks presents himself as being very much a creature of his time. We first encounter him punching numbers into his cell phone as he calls various old-age homes to ask if any of them will accept someone not yet in his 20s. He does a good job of conveying teenage boredom as he’s repeatedly put on hold before being told “No.”

So far so jaunty. Yet even as this scene unfolds, the lightly humorous tone is rhythmically punctured by glimpses of the faces we will soon come to know from Harbor Place, the retirement community that eventually accepts him. The faces belong to the old, dozing open-mouthed in chairs, shuffling dimly down pastel corridors, even (in some cases) asking to die. Human leftovers, they bravely, at times cheerfully, navigate what Philip Larkin called “the whole hideous inverted childhood,” the dubious reward that comes to those who live long enough to make the terrifying prospect of death seem almost welcome.

Despite the title, Mr. Jenks has not made himself the subject of his own film, whose stated purpose, aside from documenting the world of the assisted-living facility, is to discover whether the old have anything to teach us. The best answer to that question comes when he puts it directly to a woman named Norma: “Don’t get old, kiddo.” Another woman, asked if she has any advice for him, shoots back, “Yeah. Move out!”

Those who consider this documentary with care may conclude that “longevity” is a noun that has received undeservedly good press. A profoundly disturbing scene near the end of the film, in which an 80-year-old woman named Dotty, only hours from death, thrashes about in her bed like a trapped salmon, is enough to make the idea of an early exit look thoroughly alluring. Still, there are moments, such as “the post-bingo smoke” defiantly enjoyed by quadruple-bypass veterans, or a good joke delivered with expert timing and panache by a woman in her 10th decade, that make one feel less pessimistic.

Mr. Jenks, who also produced the film, is accompanied in his stay at Harbor Place by his editor, Jonah Quickmire Pettigrew, and his assistant director, William Godel. The team’s approach is casual and naturalistic (no attempt is made to keep cameras and microphones off-screen) and initially somewhat callow (“Look at all these old people!”), though one suspects that this is partly a clever decision on Mr. Jenks’s part to provide an emotional as well as narrative structure to his month-long stay.

Divided into four stages, each beginning with a new week, the film steadily deepens as Mr. Jenks and his crew come to know their subjects better, not just capturing their lives but forging bonds that appear to bring happiness not only to the oldsters themselves, but also to the “boys,” as the residents affectionately dub them. The film quietly implies that, even if they remain unaware of it, young people, like the old, may also suffer from a form of generational apartheid that increasingly keeps the nation’s “senior citizens” out of view, as if their mere existence were a kind of obscenity. The young are a comfort to the aged, but in a subtler way, perhaps the reverse is also true. The friendship Mr. Jenks forms with an enigmatic joker named Bill, which he seems to relish almost as much as Bill himself, suggests precisely this.

The sharpest indictment of placing parents in old-age homes is delivered, though with very little rancor, by an astonishingly peppy 95-year-old woman named Tammy. “When my mother and father were alive, do you even think we would dream of putting them in a place like this?” she asks. “Never. We took care of them. This generation can’t be bothered. Not that they can’t be bothered; they probably don’t have time. But years ago, nobody ever got sent to a place like this.”

What “a place like this” can be like is memorably dramatized on a night when a power failure in the nearby town knocks out the electricity at Harbor Place. The corridors go dark, oxygen tanks stop working, and residents “with bones like lace” fumble about in slow-motion panic as they try to get back to their rooms. As an “action” scene (for which the filmmakers must have thanked their lucky stars), it’s a stunning antidote to the adrenaline pace of the Hollywood thriller. Yet in its way, the scene is as thrilling as it is frightening. It is also educational, in that the Harbor Place staff, who are rarely seen anyway (visitors, likewise, are almost invisible), are nowhere to be found. Instead, it’s Mr. Jenks & Co. (the “little angels in the night,” as someone calls them) who come to the residents’ rescue.

In large part, “assisted living” turns out to be a euphemism for “look after one another” — which, in fact, the residents do. Self-reliance, the most traditional of American virtues, along with the ability to work together, is demonstrated here in a way that can make you proud, even as you feel ashamed of the social neglect that makes it necessary in the first place.

“Andrew Jenks, Room 335” is a young man’s film. The underlying themes are necessarily somber, but their treatment is frequently light, droll and, ultimately, optimistic. Old age may be something to be dreaded, but the residents of Harbor Place prove that it can be confronted with grace, fortitude, and, most vitally, a strong dose of gallows humor. This old-age home must have felt truly desolate, however, after “the boys” wrapped up their mission and left.

bbernhard@earthlink.net