And anecdotal evidence points to much of this use being sociable rather than medical.

When 70-year-old Robert Platshorn, a marijuana activist who was jailed for three decades after dealing the drug, moved into a gated community in West Palm Beach, Fla., three years ago, he said he “met people in my development who were looking strange at me.” Now, he said, couples invite him to their condominiums to get high together (Mr. Platshorn insisted he never accepts these offers).

Moms for Marijuana International, a pro-marijuana group that brings people together to socialize and learn about the positive aspects of the plant, has received so many queries from older people over the past year that it is creating chapters called Grannies for Grass in Illinois, Ohio and Missouri.

“There are groups out there that have trivia night and they have get-togethers,” said Vickie Hoffman, 46, a grandmother of three and a former bartender who is organizing the Missouri chapter. “It is fun, and it’s a group of great people.”

Mason Tvert, the communication director of the Marijuana Policy Project, a group that works to change marijuana laws, said he started consuming marijuana about two years ago with his grandparents, Helen and Leo Shuller, who are 82 and 88. Now, when they get together, they “have a little bit off the vaporizer,” he said, either before or after dinner, and enjoy the effects.