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You haven’t seen him in eight years. You live in different cities, have different jobs and barely keep up on social media. But when you finally meet for a drink, it takes about four minutes before you’re both laughing at a joke from 2009.
Most adults have at least one friendship like this. You may have made plenty of friends since, colleagues, neighbors, parents from your kids’ school. Yet the friends from your late teens and early twenties often feel different. Research suggests that isn’t just sentimentality.
A friendship that kept paying off
In 2017, researchers at the University of Virginia published a study that followed 169 teenagers for a decade, from age 15 to 25. Each year, participants answered questions about their friendships, social life and mental health. Their close friends were also interviewed.
The team, led by Rachel Narr and Joseph Allen, found that teens with strong, close friendships at 15 had lower social anxiety, higher self-worth and fewer symptoms of depression by the time they were 25. Being popular with a wide group of peers didn’t bring the same benefits, and was linked to higher social anxiety later.
Curiously, those benefits didn’t show up in the short term. The differences only emerged years later. As Allen put it in a press release, these experiences seem to stay with us over and above what happens afterward.
Why these years hit harder
Part of the explanation may lie in how the brain is developing. Adolescence and early adulthood are a period of major change in the brain’s social and reward circuits.
Brain imaging studies have found that teenagers’ reward systems respond especially strongly to their peers. In one well-known 2011 study led by Jason Chein at Temple University, teens took more risks in a driving game when friends were watching, and showed increased activity in the nucleus accumbens, a key part of the brain’s reward system. Adults playing the same game weren’t affected in the same way.
In other words, during these years, time with friends may simply feel more rewarding at a brain level. That intensity can help explain why friendships formed then feel so powerful.
Built from the memories we keep
Memory plays a role too. When older adults are asked to recall important moments from their lives, a disproportionate number come from roughly ages 10 to 30. Psychologists call this the reminiscence bump.
One explanation is that this is when we build our sense of who we are. First jobs, first loves, leaving home and big decisions all tend to happen in this window, and they get stored as defining memories with help from regions like the hippocampus. Friends from these years are woven into many of those memories. They were there when you were figuring out who you were.
That may be part of why reconnecting with an old friend can feel like stepping back into an earlier version of yourself.
The hours add up
There’s also a more practical reason: time. In a 2018 study, communication researcher Jeffrey Hall at the University of Kansas estimated how many hours it takes to turn a stranger into a friend. He found it took roughly 50 hours of time together to become a casual friend, around 90 hours to become a friend, and more than 200 hours to become a close friend.
In your teens and early twenties, those hours come easily. You see the same people at school, university, shared houses, part-time jobs and late nights out. Later in life, with work, family and other commitments, finding 200 spare hours for a new friendship is much harder.
Research based on phone data points the same way. A large 2016 study of mobile phone records found that people’s number of regular contacts tends to peak around age 25 and then gradually shrinks.
Shared history is hard to replace
Old friends also share something new friends can’t: a long history. They know your family, your past mistakes and the person you were before your current job or relationship. They can remind you of stories you’d forgotten and understand references nobody else would.
That shared history can make even long gaps feel small. Many people find that a friendship from their early twenties picks up where it left off, even after years apart.
Not the only friendships that count
None of this means you can’t make close friends later in life. Plenty of people form deep friendships in their 30s, 40s and beyond, especially when they share regular time together through work, hobbies or parenting.
And the research has limits. The Virginia study tracked friendships up to age 25, not across a whole lifetime, and it showed links rather than proof of cause and effect.
But it does help explain a common feeling. The friends you made during those years met you at a time when your brain was especially tuned to social connection, when your identity was taking shape, and when you had the time to build something lasting. It makes sense that they’re hard to replace.
