They are a familiar social institution, these weekend movie theater lines jammed with teen-agers waiting an hour or more to catch the hot shows of the season.

But for one film this summer, the lines have a decidedly mature look. "Cocoon," with its cast of senior citizens exploring the shift from old age to immortality, is attracting that rarest of movie audiences: older people, sometimes with their canes and wheelchairs.

In fact, according to 20th Century-Fox's marketing president David Weitzner, the film has attracted such a marked share (60 percent) of older moviegoers -- in movie parlance, older means 25 and over -- that its advertising was modified recently to lure more of the youth audience.

"Cocoon's" television ads, for instance, now emphasize the picture's cast of aliens, which they did not initially. The extraterrestrials' mother ship and other hardware devices are now shown along with a glowing hand on a doorknob and the blinding light underneath the aliens' full-body masks -- all of which "would appeal to a younger audience," according to Weitzner.

But what is attracting their grandparents?

Why is the older generation suddenly willing not only to go to the movies but often wait in long lines to do so? In record-setting heat, no less, and at theaters that do not feature senior citizen discounts.

"I came because it's not about teen-agers," said Fred Robertson, a 55-year-old insurance underwriter. "There are teen-agers in the movie but it also has older people in it, and we like that. It portrays older people in a good sense. For once they're doing something good about middle age."

"We relate to the cast," said 63-year-old Helen Schultz, as she was about to watch the film with her 61-year-old friend Tess Joseph. "I get to see Don Ameche again, and I like Hume Cronyn. We seniors are a big group and a lot of us have time and money to spend on movies. But usually they don't make anything we want to see."

Many older moviegoers, however, indicated that the age of the cast members or the picture's Fountain of Youth plot had nothing to do with their choice.

Eighty-two-year-old Florence Katz, for one, said she came because her granddaughter brought her.

"I enjoyed 'Cocoon' very much," she giggled on her way out of the theater. "I have arthritis and so I liked seeing the cast walk nicely, which I can't do so well [more laughter]. . . but I didn't come because there were old people in the movie. I came because I love to be with young people."