We’ve all heard it a hundred times: they grow up so fast.

But as a new parent, you shrugged it off. Even when interacting with seniors, you never believed you’d be an old timer yourself one day. It couldn’t happen to you.

You’re sailing through life and then…wham, old age hit like a thief in the night the day of your youngest child’s 30thbirthday.

And as the candles atop the cake are lit, you’re overcome with a wave of memories—and because you’re a good parent you don’t regret any of them.

As you flash back to the day your child was born, you’ll no doubt start questioning yourself, wondering where your life went?

Kids grow up, we all know it—after all, time moves forward not backward. But why does it all happen faster as you get older?

While the good news is people are living longer and having more of their life in better health than before, the bad news is time does seem to pass faster and the reason isn’t that life is cruel.

Of course, aging doesn’t grant us the power to disrupt the space-time continuum, so it’s not a real problem. But why do we perceive it to be?

Here are a few of the theories.

You don’t have as much dopamine anymore

If there were a superstar among brain chemicals, it would be dopamine.

In the brain, dopamine is heavily involved with concentration and memory, both of which are involved in estimating the flow of time.

According to science, dopamine levels fall by up to 10% every decade throughout adult life. At least one study says this helps explain why time seems to go by more quickly as we get older.

The study was published online in the journal Arquivos de Neuro-Psiquiatria

A team of academic researchers from the Sao Jose Faculty of Medicine in Brazil confirmed that time seems to speed up with age by asking 233 men and women between 15 and 89 to close their eyes and count the passing of 120 seconds.

All ages perceived the two minutes as passing more quickly than it actually did. But the oldest group judged time as passing 25 percent more quickly than the youngest group.

While it’s a big leap to prove that lower levels of dopamine influenced the study participants’ perception of time, the researchers don’t think it is a coincidence.

You spend less time acquiring new skills

In addition to age-related changes in levels of dopamine, some scientists believe time seems to speed up as we get older because we spend less time acquiring new skills and become bored with routine tasks.

“The time it takes to learn something new is always subjectively prolonged, such as the first sexual relationship, the first job, the first trip without parents or the first experience of living away from home,” the Brazilian team said.

Just think about that for a second.

Everything was new when you met your spouse and you have many wonderful memories. Your engagement seemed to stretch on for an eternity and it’s easy to recall the things you did together. But if you are average—and chances are real good that you are—you were really engaged only about a year before getting hitched (average is 14.5 months). 

The same is true with your children. The early years were full of new adventures and went by slowly—but the later years flew by in a blur.

You want to make the best of what time you have left

Another theory suggests our race against the clock to get more done (even the simple task of counting seconds), our need to cram as much into each hour as humanly possible — like answering text messages and eating lunch while attending a meeting — causes life to speed up.

Time relative to our age becomes smaller

But perhaps the best explanation of why time flies as we age has to do with proportions—or the idea that we perceive a period of time as the proportion of time we have already lived.

First proposed in 1877 by French psychologist Pierre Janet, the “ratio theory” suggests that we are constantly comparing time intervals with the total amount of time we’ve already lived.

To a two-year-old, for example, a year is half of their life. Which is why it seems an extremely long period to wait between birthdays when you are young.

To take that a step further, when you were five one year was 1/5thof your life. At 60, however, one year is only 1/60thof your life. So the time between birthdays seems to get shorter and shorter, as if time is speeding up.

But your birthday, of course, always comes around at exactly the same interval.

Stop worrying about it

What does all this mean?

That’s a good question. To me it means you should stop worrying about growing old. We’re all doing it. Relax and know that although it doesn’t seem like it sometimes, time is moving at the same rate today as it did when lazy summer days seemed to stretch on forever during our childhood.

Here’s the thing: At the heart of why everything seems to move faster as we age is the assumption that death is inevitable.

But I should point out that according to science, reversing or slowing the aging process down is a technological challenge, not a physical impossibility. Thanks to progress on numerous medical fronts, we’ve gotten better at fighting off disease in recent decades and death rates have dropped.

And in case you needed something to grab your attention, here it is: In December of 2017 the digital company Inverse pointed us to the profile of Dr. Aubrey de Grey, co-founder and chief science officer of the SENS Research Foundation, an organization whose mission is to find a way to extend the healthy human life span by hundreds of years.

Through his research, de Grey continues to make headlines. Humans will “definitely” live much longer, he said. And the first person to make it to 1,000 years is likely living today.

“I can’t see how it could not be,” de Grey recently told Kira Pelkoff, editor-in-chief of online science magazine LeapsMag, during an interview. “I would say people in middle age now have a fair chance.”

Okay, that last statement is almost embarrassing, but it’s wildly interesting. And even though it might sound like a sci-fi movie, a lot of people are cheering de Grey on.

Like him, world famous (and famously wealthy) bigwigs like Oracle co-founder Larry Ellison and Amazon’s Jeff Bezos, and others like them, believe death will one day be optional. In a quest to get there as fast as it can scientifically and medically be done, they’re investing hundreds of millions of their own dollars. 

So there you have it. If Silicon Valley says it is, we have to believe, right? Well, maybe. And that’s a big maybe.

But in the meantime, no matter how much you dislike dealing with the aching joints, sore back, and failing eyesight that come with aging, I know from personal experience that worrying about it does no good, because, well, there’s really nothing you can do about it, except, well, hope modern science is on the right track.

You’ll also want to hope that science’s focus is not just on lifespans—how long we live—but also on healthspans, or how well we live.

So for now, stop being desperate about your age and start living. Your child’s birthday should be a personal holiday, a time to celebrate what you’ve accomplished instead of fixating on how much time you have left.

At the end of the day, worrying about getting older is worse than actually getting older. It makes the years seem to fade into memory more quickly—like dandelions in the wind.