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At some point, you've probably heard your family or neighbors mention how time flies as you get older. Maybe you've even looked back on the last few years and noticed it yourself.

Like, there's no way you just graduated. Freshman year feels like it was a few months ago. The idea that time passes more quickly as you age is a pretty common one, and there are a lot of potential reasons it might happen.

But the good news is, no matter what trick of the brain is causing this phenomenon, you can probably do something to stop it. There are all kinds of ways we measure the passage of time. One is time estimation, or how fast the clock seems to be ticking.

Researchers test this by asking people to guess when a minute has passed, or things like that. The one that's most relevant to why time flies as you get older, though, is time perception. This is how psychologists describe how fast time seems to pass when we think about events.

Multiple studies have shown that things like our time estimation ability don't change over the course of our lives. But time perception does. In one 2010 paper from the journal Acta Psychologica, researchers reviewed previous studies and then conducted two new ones of their own.

In total, they analyzed survey results from almost 2000 participants, who were anywhere from 16 to 80 years old. In one experiment, participants were asked to rate how fast certain periods of time had gone on a five-point scale, from “very slow” to “very fast." The researchers found that, when asked how long the last hour, month, or year had gone, all participants said it passed at about a normal speed, no matter their age. The real difference came when participants were asked how long the last decade had gone.

It wasn't a dramatic difference, but the older people got, the more quickly they tended to say time had passed. Still, it's not clear what would happen to cause this shift in perception. Some hypotheses say that you had more new experiences as a kid, like your first lost tooth, or your first soccer game.

So there's more to remember from back then. Meanwhile, your adult life feels a lot less novel and, as one paper put it, “grows hollow and collapses.” Which is… lovely. Another idea is the ratio model, which says that time ticks by more slowly when you're younger because you haven't been around as long.

Like, when you're eight, a year feels like forever because you've only been alive for eight years. But when you're eighty, a year isn't as significant in the grand scheme of your life. There's also a hypothesis called forward telescoping, which describes how we tend to underestimate how long ago important things happened.

It's why you can't believe High School Musical actually came out over ten years ago, and why you suddenly feel really old when you think about it. Right now, all of these ideas have some evidence to support them, but there's not a real winner yet. They're also difficult to test in a lab, so we may never be able to conclusively prove any of them.

But there is one, newer idea we could prove, one that suggests that time whizzes by as you get older because of how your body chemistry changes. See, as you grow up, your body produces less dopamine, a neurotransmitter that's usually associated with things like motivation and reward. But, at least in mice, research has shown that the amount of dopamine in the body can also affect time perception.

In a 2016 study, published in the journal Science, mice were taught to use sensors to signal whether the interval between two sounds was longer or shorter than 1.5 seconds. Originally, all of the mice were really good at this. But when they had their dopamine-producing cells stimulated, they tended to think that interval was longer than it really was.

And when they had those cells suppressed, they thought it was shorter. According to the researchers, this suggests that extra dopamine causes mice to experience time as passing more slowly in general. So maybe this could also apply to humans.

There's already some evidence: Parkinson's patients, who typically have low dopamine levels, have been shown to underestimate how fast time is going, like if they're asked to reproduce the tempo of a piece of music. More studies about long-term time perception in these patients would be helpful. But in general, it's a promising idea, and one that might be easier to research in a lab.

Even though we don't know exactly why this time phenomenon happens, there could be some ways to stop it from happening, depending on what's causing it. Like, you could try more new things, so that you have more to reflect on later. Or you could do more things that are really exciting and fun for you, so that your dopamine levels are generally higher.

You could also just accept that High School Musical really did come out over a decade ago, and so what if you're a little older. There's no guarantee that any of these things will make your life seem longer, but a life full of new and fun experiences definitely can't hurt. Thanks for watching this episode of SciShow Psych!

If you love learning about the world as much as we do, you can help us introduce science to the next generation by checking out SciShow Kids. It might not make your life pass more slowly, but it will hopefully inspire curiosity and excitement in the world's future psychologists. You can visit the channel and subscribe at youtube.com/scishowkids. ♪ OUTTRO ♪.