Today, members of the millennial generation are ages 23 to 38. These ought to be prime years of careers taking off and starting families, before joints really begin to ache. Yet as a recent poll and some corresponding research indicate, there’s something missing for many in this generation: companionship.

A recent poll from YouGov, a polling firm and market research company, found that 30 percent of millennials say they feel lonely. This is the highest percentage of all the generations surveyed.

Furthermore, 22 percent of millennials in the poll said they had zero friends. Twenty-seven percent said they had “no close friends,” 30 percent said they have “no best friends,” and 25 percent said they have no acquaintances. (I wonder if the poll respondents have differing thoughts on what “acquaintance” means; I take it to mean “people you interact with now and then.”)

In comparison, just 16 percent of Gen Xers and 9 percent of baby boomers say they have no friends.

The poll, which looked at 1,254 adults 18 and up, did not report results for the up-and-coming Gen Z (who report high levels of loneliness on other surveys), or for the oldest adults in the country. And we should note: Loneliness tends to increase markedly after age 75; social isolation among the elderly remains a huge problem that will only grow worse as baby boomers age. So perhaps it’s not the case that millennials are the loneliest of all.

Still, the findings on millennials are surprising. Why do a fifth of these 20- and 30-somethings say they lack friends? YouGov’s poll didn’t measure why.

If this generation is truly lonelier, that’s concerning for a number of reasons: Research shows that loneliness tends to increase as we get older. What will happen to millennials, who are already reporting high levels of loneliness, when they reach old age?

It also raises the question of whether everyone who’s lonely, millennials included, is more isolated from spending more time on the internet. (Though there’s also evidence that the internet can help lonely, isolated people connect with others.)

But while there may be something particular happening with millennials, it’s also possible loneliness naturally ebbs and flows throughout life. A 1990 meta-analysis (a study of studies), which included data on 25,000 people, found that “loneliness was highest among young adults, declined over midlife, and increased modestly in old age.”

So there may be a cycle and it might not be new. Many 30-somethings find it gets harder to make new friends as they age. Friends move, family and work obligations increase, and plans to reconnect get perpetually booted into the future on email chains.

More recently, in a 2016 paper, researchers in Germany found a peak of loneliness in a sample of 16,000 Germans at around age 30, another around age 50, and then increasing again at age 80.

“We don’t quite know why this is happening,” said Maike Luhmann, a psychologist who researches loneliness at Ruhr-Universität Bochum and co-authored the paper. “So most of the previous research has focused on old age, and for good reason, because it’s when loneliness levels are high.”

The bigger point, she said, was “researchers have ignored that loneliness can happen at any time.”

And that’s important. Because loneliness is bad for our health.

Loneliness is associated with higher blood pressure and heart disease — it literally breaks our hearts. A 2015 meta-review of 70 studies showed that loneliness increases the risk of dying by 26 percent. (Compare that to depression and anxiety, which is associated with a comparable 21 percent increase in mortality.) There’s evidence that chronic loneliness can turn on genes involved with inflammation, which can be a risk factor for heart disease and cancer.

Make no mistake: We need stress. We need some amount of loneliness. The pain of loneliness is a reminder that we are social creatures who need other people. It’s also important to recognize that loneliness isn’t the same as having few friends. It’s the perceived social isolation that harms us. We can certainly have fulfilling, protective relationships with just a few people.

“As long as we then do what we should do — reconnect with people — then loneliness is a good thing,” Luhmann said. “It becomes a bad thing when it becomes chronic. That’s when the health effects kick in. And it becomes harder and harder to connect with other people the longer you are in the state of loneliness.”