The millennial cohort is roughly defined as those who were born in the 1980s to the early 2000s — although there is some debate about the boundaries. Millennials, due in part to their digital savvy, already are credited with significant changes in how we live, work and interact.

But what is particularly striking is how quickly the cohort has rewritten the rules for courtship, sex and marriage. In 2018, the median age of first marriage was approaching 30 (29.8 for men and 27.8 for women). T hat’s more than a five-year delay in marriage compared to 1980, when the median age was 24.7 for men and 22 for women.

A 2017 study in the Archives of Sexual Behavior found that many younger millennials in their early 20s aren’t having sex, and are more than twice as likely to be sexually inactive than the previous generation. Another study found that American couples ages 25 to 34 spend an average of six and a half years together before marrying, compared with an average of five years for all other age groups.

Critics say digital saturation has made millennials more socially isolated, restless and entitled, which could explain why they are having less sex than earlier generations. And when millennials do have sex, it’s often viewed as less meaningful because they engage in “hookups” or sexual relationships described as “friends with benefits.”

Dr. Fisher, author of “Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray,” has devoted her career to studying love and relationships. Most recently she has collected data on more than 30,000 people related to current courtship and marriage trends. Dr. Fisher believes that instead of criticizing and judging millennials, perhaps we should be paying more attention. It’s possible, she said, that today’s singles are carving a more successful path to lasting love than previous generations.