In the US turning 18 is arguably the most important age in our lives, both culturally and legally.

But have you ever wondered: why is 18 considered an adult?

So around the world 18 is a momentous age and the US is no exception.

At 18 US teenagers can vote, enlist in the army, work full time, sign contracts, buy tobacco, and move out of their parents' house.

But even though we think of 18 as the magic number when kids suddenly grow up, how did we settle on this age as opposed to others?

You might think it's because of some sort of physiological explanation.

Well kinda, but not really.

While most people are done with puberty by 18, many people are not.

Plus recent studies have found that people's brains are still growing and maturing until age 25.

And let's face it: people who can't rent a car don't always make the most adult decisions.

Another easy explanation could be that 18 is considered an "adult" because of long standing tradition.

But that's also not really true.

In fact 18 as the legal, nationwide voting age wasn't enshrined in the US constitution until 1971.

And until World War 2, the draft age was 21.

Additionally, outside of the US coming of age ceremonies vary pretty considerably on the official age of adulthood.

So how did we arrive at 18?

Well to answer the question of how and when we started thinking of people as "adults" we actually need to reverse our thinking and ask: how did the concept of a "childhood" emerge?

And shift from a mere fact of life to a whole category of people that need special rules?

At the beginning of the 16th century, kids as young as 7 were expected to work and were often being prepared for adult labor.

However, most of this work took place either within the home, as agricultural labor, or during apprenticeships were kids learned a ú trade.

The shift of kids to a protected class really kicks off during the industrial revolution.

It wasn't that children were necessarily working in greater numbers, but rather that the rapid spread of industrialization altered the quality, duration, and danger of child labor.

In the 18th century and going into the 19th century, poor children were working in factories outside of their home and without parental supervision.

They were also working for more hours without the seasonal breaks that were sometimes a part of farm labor.

Plus they were working in overcrowded factories that had poor living conditions, unsafe practices, and very low wages.

Yeah, so not great to be a kid.

By the early to mid-19th century we see more activists across Europe and the US increase their efforts to protect and "save children" from the horrors of abuse and poverty.

This is mirrored the rise in novels about scrappy young orphans with soot-covered faces like Oliver Twist, Great Expectations and Les Miserables.

And, shoutout to the surprisingly accurate American Girl Doll Samantha.

In the 18th and 19th century we see an increase in the spread of education for young boys, not just the wealthy.

And by the early 20th century we start to get laws in Europe and the US making schooling mandatory up to certain ages like 16 or, you guessed it, 18.

In 1938 Congress passed the Fair Labor Standards Act, which set the minimum work age at 16 during the school year, 14 for after school jobs, and 18 for dangerous jobs.

So the 19th and 20th centuries were big turning points for the concept of kids just being kids.

But it's important to note these 19th century evolutions weren't extended to all children.

Scholar Robin Bernstein notes in her work Racial Innocence that Harriet Beecher Stowe's 1852 abolitionist novel Uncle Tom's Cabin presents us with two child characters Eva and Topsy.

White Eva is represented as having the pure hearted innocence of childhood, while black Topsy is represented as an out of control child who has been corrupted into bad behavior because of the evils of slavery.

So it's Topsy's link to the forced labor and the attendant ills of slavery that 'corrupts' her in Stowe's abolitionist novel, requiring that she be "saved" and educated both formally and through her proximity to Eva.

Uncle Tom's Cabin is also contemporary with 19th century debates on child labor for European and white American children.

Yet false arguments that represented minority groups as "perpetual children" were often utilized as a justification for enslavement and forced labor.

So childhood as a category of protection from work was often in flux.

So that leaves us with our final question: how did 18 become the legal crossroads between kids and adults?

Well, part of the problem with defining an "adult" in modern Western society is the lack of formal initiation rights.

Many societies throughout history have had a very well defined rituals like the Hamar Cow Jumping, Apache Sunrise Ceremony or Japanese See Gin No Hi that conferred adult status and responsibilities on those who went through them.

In the US the closest approximation we have to formally granting adult status is the ability to vote, which technically makes you a full citizen.

And up until the 26th amendment, the voting age was a state by state decision.

But the idea of a national voting age at 18 really begins with our good friend Franklin Delano Roosevelt.

During WW2 FDR lowered the draft age from 21 to 18.

But the voting age for national elections remained 21 in every state.

This outraged young men sent off to war, popularizing the slogan "Old enough to fight, old enough to vote."

And some states agreed.

Georgia was the first state to lower the voting age to 18, in 1943.

But many kept the voting age at 21.

This WW2 slogan and debate flared up again during the draft in the Vietnam War, because Baby Boomers weren't too crazy about being sent into danger if they couldn't have a say in the elected officials shipping them there.

But it was a fight to get it to 18.

In 1969, 60 resolutions to lower the voting age were put up and rejected by congress.

Fortunately in 1970 an amendment was added the Voting Rights act of 1965 that established 18 as the voting age.

But even that was challenged in the Supreme Court Case (Oregon vs. Mitchell).

Which gave the wonky result of the Federal voting age being 18, but states still having the power to make their local elections 21.

Fortunately, it was finally resolved when the 26th amendment was passed in 1971, which lowered the voting age in all elections to 18.

Although it's not always framed this way, the fight surrounding the voting age and the age of majority also somewhat mirrors other civil rights discussions of the 20th century.

Debates about the voting age were also occurring alongside other debates like the Voting Rights Act of 1965, which extended voting protections to African Americans and underserved communities, and the increased exposure in the press of movements such as the gay liberation movement and women's rights.

All of these centered on the key question of not only how old you had to be to be considered an adult, but who would be recognized as a full citizen under the law.

So how does it all add up?

18 may the age we've settled on as adult, but it isn't for entirely natural reasons.

It seems that 18 has more to do with the combination of the industrial revolution and the development of labor laws, FDR lowering the draft age during World War II, and the corresponding fight for full citizenship.