“Kids Grow Up Too Fast” Isn’t Just a Feeling: Here’s the Science Behind It

“They grow up so fast.” Every parent on the planet is familiar with this deep-seated sentiment. You’re putting away clothes size 3T, yet it feels like just yesterday you brought them home from the hospital. You watch them enter high school, vividly recalling their first bike ride fall… No, you’re not crazy: this feeling is backed by science. Mathematicians, philosophers, physicists, neuroscientists, and psychologists worldwide have all studied this universal sensation among parents. The explanations are many and converge in fascinating ways.

First, consider the proportionality theory. Research by biomathematician Christian Yates, building on the work of French philosopher Paul Janet, indicates that our brains unconsciously gauge time relative to the span of our lives. For a 5-year-old, one year is 20% of their entire existence—a significant chunk of everything they know, hence the seemingly endless wait between birthdays. For a 40-year-old parent, however, that same year is merely 2.5% of their life, significantly less impactful. This principle isn’t limited to parents, which is why people often feel that “life is short.”

Beyond the numbers, our biology plays a role as well. In a study titled Why Days Seem Shorter as We Age, physicist Adrian Bejan explains that our minds measure time through a series of mental images. A young brain processes information much faster than an adult brain. It’s as if childhood is recorded in “slow motion” at 120 frames per second: each day is packed with vivid details, whereas adults might capture their life at 24 frames per second. As we record fewer new images over the same period, it feels as though the movie of our lives is speeding up.

This concept is also supported by neuroscience. Researcher David Eagleman has shown that our brain gauges the passage of time by the amount of new memories formed. Unlike childhood, which is filled with continuous “firsts,” adulthood quickly settles into routine. The brain automates most daily tasks to save energy. When adults look back, they find fewer new, significant memories, leading them to conclude that “time has flown by.”

Finally, beyond our internal perception, there’s a very real societal factor at play. In her book Your Children Are Not Little Adults, clinical psychologist Béatrice Copper-Royer discusses the shrinking duration of childhood. Early exposure to screens, a constant flow of information, and the modern tendency to treat children like mini-adults disrupt their development. Seeing an 8-year-old mimic the behaviors of a 14-year-old confirms parents’ feelings: the era of carefree youth is noticeably shorter. Ultimately, if time is passing so quickly with them, it might be the most beautiful sign that it’s time for us to slow down ours.