It happened about a year ago. I stepped off the subway and spotted an ad on the station wall for a food delivery service. It read, “When you want a whole cake to yourself because you’re turning 30, which is basically 50, which is basically dead.”

After a bunch of us squawked about the ad on social media, the company apologized for what it called attempted humor and what I’d call ageism.

Such a jab constitutes a mere microaggression compared with the forms ageism often takes: pervasive employment discrimination, biased health care, media caricatures or invisibility. When internalized by older adults themselves, ageist views can lead to poorer mental and physical health.

“It’s an incredibly prevalent and insidious problem,” said Alana Officer, who leads the World Health Organization’s global campaign against ageism, which it defines as “stereotyping, prejudice and discrimination” based on age. “It affects not only individuals, but how we think about policies.”

As a first step in the campaign, announced in 2016, WHO has invested $500,000 in research. Four teams around the world are collecting and assessing the available evidence on ageism — its causes and health consequences, how to combat it and how best to measure it.

Their work will appear in a U.N. report to be published within a year and will culminate in international mobilization, organizers hope.

One of the research groups, at Cornell University, has completed its task, and published its study last month in the American Journal of Public Health. It brings surprisingly good news.

The team spent a year and a half sifting through dozens of articles, from the 1970s through last year, evaluating anti-ageism programs. Such efforts popped up around the country in the years after psychiatrist and gerontologist Dr. Robert Butler coined the term ageism in 1969.

“But are they doing any good?” asked Karl Pillemer, a gerontologist and senior author of the study. “Do interventions that purport to change people’s attitudes about ageism actually work?”

The researchers analyzed 64 studies, most conducted in the United States, involving 6,124 participants, from preschoolers to young adults. The investigators classified about a third of the programs studied as intergenerational, meaning they created contacts between young and old that, in theory, could lessen prejudice.

Another third or so were educational, teaching facts about aging as a way to challenge stereotypes and myths. The remainder combined both approaches.

These were small, inexpensive, local efforts, pointed out the study’s lead author, David Burnes, now a gerontologist at the University of Toronto. They included:

>> A program in which undergraduate psychology students corresponded with older adults by email, developing deepening relationships over six weeks.

>> A gardening project that brought fourth graders to a Tennessee senior center twice weekly for a month.

>> A four-session program in an Australian high school, incorporating discussions, games and role-playing about aging and adult development.

Almost universally, after such interventions, participants showed significantly less ageism on attitude tests and greater knowledge of aging than comparison groups that hadn’t taken part. The combined educational and intergenerational approach proved the most effective.

“The message is loud and clear,” Pillemer said. “Ageist attitudes don’t seem as baked in as we think. They may be relatively malleable.”

That matters because ageism is hardly benign. “These stereotypes can have direct impact on older people’s health and function,” said Becca Levy, a social psychologist at the Yale School of Public Health and leader of the WHO-sponsored review of studies on health consequences.

The research her group is reviewing will include her own important work on ageism, conducted over 20 years. Levy has shown that older people who see aging in positive terms are much more likely to recover from disability than those who believe negative age stereotypes.

They’re also more likely to practice preventive health measures such as eating well and exercising. They experience less depression and anxiety. They live longer.

Recently, Levy and her colleagues have been looking into ageism and cognition.

“With negative stereotypes, older people have a higher risk of dementia,” she said. “They have greater accumulations of plaques and tangles in the brain, the biomarkers of Alzheimer’s disease, and a reduced size of the hippocampus,” the part of the brain associated with memory.

So this is no joke. Yet “there’s a lot of social acceptance of ageism,” Levy noted, pointing to television, social media and everyday interactions. Although studies have found that children as young as 3 or 4 hold ageist ideas, now “we have research showing that we can overcome it.”

Key questions remain unanswered. The studies the Cornell group analyzed followed participants for an average of 15 weeks, so we don’t know how long the positive effects of such interventions last. There’s scant data, too, on how to shift older people’s own internalized ageism.

Nor do we know whether and how positive attitudes translate to action. Will less ageist citizens support stronger enforcement of laws against workplace age discrimination? Or defend Medicare and Social Security from heedless budget cutters?

It’s not always easy to find the balance between shrugging off offensive messages and counterproductive scolding, but individuals can speak up about ageist generalizations.

We can argue the merits of one or another politician without rejecting candidates simply because they’re too old (or too young). We can distribute atta-girls and atta-boys to those unafraid to show their true faces and hair color (while acknowledging that, yes, the labor market sometimes dictates otherwise).

We can gently protest when even beloved friends and family succumb to stereotypical thinking.