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How do you define adulthood? It's a difficult question because that delicate brain of yours stays squishy well after you start paying your own rent.

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[Motif from theme song plays].

If you turned 18 in the United States... Congrats, you're legally an adult.

Even if you're not doing your own laundry yet... You're not your parents' responsibility anymore. [I'm sure they're celebrating too...] But, here's the catch. Even though you reached adulthood on paper;.

Your brain still has more work to do. Ask a neuroscientist, and they'll tell you that people don't get to the so called "adult" brain until their thirties. Or maybe even never.

That's because your brain never stops changing (even after you "grow-up"). So, really, there's no such thing as a fully developed brain! Scientists are still learning a lot about brain development, but they've come a long way.

We used to think that you were born with all the neurons you would get. But, now we know that new neurons and other brain cells can grow well into adulthood! And the networks of connections between those neurons -- [which] let you learn, [and] think, and do things -- is also in flux.

So, your brain never really stops changing. But, we do know it goes through developmental stages. After all, baby brains are really different from teen brains, which are really different from the brains of middle aged people.

In terms of volume, your brain is full sized by the time you're ten, or so. You're born with most of your neurons, so in your first few years you're adding a lot of glial cells. Brain cells that help support neurons by feeding, insulating, and protecting them.

But it still has a lot of structural changes to under go; like a huge shift in the ratio of gray to white matter. Gray matter makes up the bulk of neurons while white matter is the stuff connecting it all together. It's made of thin nerve fiber called axons surrounded by myelin -- a fatty substance that electrically insulates axons and speeds along brain signals.

Now, both of these are important, but as kids start getting angsty (and move into their teenage years); they trade more gray matter for white matter. For one, the brain is laying down more myelin... Which adds to white matter and lets neurons send information more quickly.

On top of that, the brain is also going through a kind of intense spring cleaning, and getting rid of connections that aren't being used. This process is called synaptic pruning. It sounds a little weird because we tend to think more is better.

So how could fewer brain cells be a good thing? Well, synaptic pruning is thought to make the brain more efficient. There may be fewer neurons, and fewer contacts...

But that allows the brain to make better, stronger connections between different parts. Scientists don't fully understand what these structural changes do, but they think that's one reason why you can eventually handle more complicated ideas. Like, linear algebra (instead of just addition).

And these structural changes don't happen at the same rate for the entire brain. We know that the visual center in the back of the brain seems to have things mostly wrapped up by the time you're 20 years old. But it takes [at least] another decade for the frontal lobe -- the part of the brain responsible for decision making and problem-solving (among other things) -- to do the same.

There are also other ways for the brain to change as you grow up. Like, the number of neurotransmitter receptors on the brain cells, how good your brain is at using sugar, and [of course] hormones. [Can't forget hormones]. But, these aren't very well studied.

We do know a couple things about teen brains and the behavioral transitions to adulthood, though... Neuroscientists are finding that teen and young adult brains can function pretty well, but not necessarily all the time. If you give teenagers cognitive tests (for instance), they can usually measure up to older adults, but not if they're scared.

In one 2016 experiment, researchers quizzed volunteers in three age ranges (13 to 17, 18 to 21, and 21 to 25) and told them they might randomly blast a loud sound into the room. While slightly older adults were able to mostly ignore the threat of the noise... Teenagers and young adults were thrown off.

Scientist have found similar things when they give teens and young adults happy or sad emotional cues. Which could mean [as cliché as it sounds]. Teenager's brains aren't quite as good at regulating and processing emotions yet.

Which could explain why some teens take more risks than older people. And, because those experiments involved [college aged] young adults too. It seems like this problem [if you want to call it that]...

Doesn't magically go away once you turn 18. Neuroimaging studies suggest that this behaviour might be linked with be linked with less activity in the prefrontal cortex. It's possible that the brain circuits that prevent emotions from overwhelming your ability to make decisions.

Don't fully develop until you're a little older. Some researchers think that the definition of adulthood should really be when you've got more control over these kinds of emotional responses. Which for some people might be 22.

But, for others, could be much later. As you get older and learn new skills you're strengthening connections and physically rearranging things. So, even if you're over 18 and don't feel quite like a real adult yet.

Take heart that your brain has more changing to do. Thanks for watching this episode of SciShow Psych. If you want to learn more about the practical side of adulting.

Go check out how to adult at Which has videos about everything from doing taxes to picking wine. [Theme song].