You’re 18? Congratulations, You’re an Adult. 18 Was Our Best Guess. Terms Apply!

My sister was 17 years and 364 days old when we went to file some legal documents, but she was still a teenager. 24 hours later, her files were accepted and she got her ID. YAY!!! 

She went back home, excited to finally unlock new life quests; driving a car, getting an income, being independent… But I was intrigued, to say the least. I had a few questions in mind, but the most prominent one was certainly: WHAT ACTUALLY CHANGED? 

See, my sister didn't magically gain extra brain cells (although I would've loved it if she did), or accumulate life experience, or even change her personality (or stupidity, love u sis!), and yet, society has crossed a legal boundary. 

We generally treat 18 as adulthood almost automatically, but a question almost everyone never dares to ask is why 18? Not 19, or 17, or 21? And why not an individualized assessment of maturity? Why should adulthood be understood as a single threshold at all? 

Defining Adulthood 

Before asking the not-so-famous question: why 18? We should establish what we are measuring, because there are different types of maturity (the more you know!). 

Adulthood is a surprisingly broad term, so we are going to break it down into different concepts: 

● Biological maturity 

● Cognitive development 

● Emotional maturity 

● Independence 

● Legal responsibility 

● Political autonomy 

● Social adulthood 

And because they are different, someone can have one without necessarily having all the others; I could be independent without necessarily being 18, and I could be biologically mature and cognitively developed but not as independent.

This realization immediately complicates the idea that there should even be one age at which everything changes, so now the question becomes: If adulthood consists of several different forms of maturity, why should one number define all of them? 

Perhaps the problem is that we are asking one number to do a job that belongs to several different concepts.

The History Behind Choosing 18 as the Age of Adulthood 

If we want the literal answer to our question, we should investigate the history behind it. Historical majority ages varied throughout the years and with each civilization, so to simplify, we are going to look into the 17th century and later: 

From the 17th century through the mid-20th century, 21 emerged from English common law as a standard age for full legal adulthood, inheritance, and military readiness. This standard influenced later legal systems, including the United States, where 21 remained an important age threshold for legal adulthood and several rights and responsibilities (Cornell Law School). 

During the 20th century, the case for lowering certain age thresholds became increasingly prominent. In the United States, the traditional voting age of 21 was challenged as young adults were expected to assume responsibilities such as military service while being denied the right to vote. The Vietnam War intensified this debate, contributing to the political movement behind the slogan “old enough to fight, old enough to vote.” Congress lowered the voting age to 18 for federal elections in 1970, and the Twenty-Sixth Amendment, ratified in 1971, constitutionally prohibited governments from denying or abridging the voting rights of citizens aged 18 or older on the basis of age. However, this change concerned voting specifically; it did not establish 18 as a universal threshold for every right and responsibility associated with adulthood. 

According to the UN Convention on the Rights of the Child, Article 1, “for the purpose of the present convention, a child means every human being below the age of 18 years unless the law applicable to the child, majority is attained earlier” (UNICEF). The Convention therefore established a widely used international definition of childhood, while allowing national law to determine when majority is attained earlier (UNICEF). This is important because it shows that 18 was not chosen because science identified it as a precise biological threshold, but rather became a practical legal boundary for determining when people could take on certain rights and responsibilities in society. So, did 18 become the age of adulthood because it represents a scientifically identifiable stage of human development – or because societies gradually converged on it as a practical boundary? 

18 and Developmental Science

Developmental science is the study of how humans develop across their lives (smart, I know!). But it explores different facets to determine human functioning: 

● Executive functioning 

● Impulse control 

● Risk assessment 

● Decision making 

● Susceptibility to peer influence 

● Long-term planning 

● Cognitive development 

We all know the argument that human brains do not fully mature until their mid-20s, usually around age 25. But the actual science is a little more nuanced than this. 

Adolescent brain development is not a process in which the brain suddenly becomes “adult” on a particular birthday. Instead, different brain systems and cognitive abilities develop along different trajectories and at different rates. Executive functions such as working memory, inhibition and cognitive control continue to strengthen throughout adolescence and into young adulthood (Best and Miller). The prefrontal cortex, which plays an important role in planning, cognitive control and goal-directed behaviour, undergoes prolonged structural and functional development during adolescence (Blakemore and Choudhury). 

At the same time, this does not mean that adolescents are simply incapable of rational decision-making until they reach some later age. Research shows that adolescents can perform many cognitive tasks at adult-like levels, while their decision-making can become less consistent under conditions involving strong emotions, social pressure or immediate rewards (Crone and Luna). In other words, maturity depends not only on whether a particular brain region has developed, but also on the situation in which a person is making a decision.

The visual below illustrates some of the major patterns observed in adolescent brain development:

And this table summarises the maturity timeline:

So different aspects of maturity develop at different rates; some mature early and others late, making a single birthday of 18 years an imperfect representation of individual capacity. And if so, we run into a problem: what would replace 18? 

Let's say 18 is arbitrary and we try using another age, for example 21, like in the 17th to mid-20th century. Then we run into the same problem: Why 21? What about 20, 19, 25 or 30? And what if two 18-year-olds have radically different levels of maturity? Should law assess them individually? 

Obviously, that creates enormous problems. A legal system needs predictability, consistency, enforceability, equality before the law and clear boundaries. UNICEF's discussion of evolving capacities, especially the tension between individual developmental differences and the practical necessity of age thresholds, leads to an important realization: The fact that 18 is imperfect does not necessarily mean that using it as the age of adulthood is irrational! Perhaps legal boundaries are inevitably imperfect (UNICEF). 

The Strange Part: Adulthood Already Has Multiple Birthdays 

Wait, WHAT?

Yes, it does. Ironic, isn't it? Before doing my research, I never thought people as old as me could drive cars in the US. (Yes, I am 16, and yes, I have never driven a car in my entire life! Pretty cool giving commentary about the US when I'm Moroccan.) 

But different legal systems assign different age thresholds to different rights and responsibilities; some countries allow voting at 18 but drinking at 21, while other countries offer you the whole bundle on your 18th birthday. 

So society already recognises that different capacities can have different thresholds. And so if we don't believe someone becomes capable of everything simultaneously, why do we need one universal concept of adulthood? 

I asked my sister, who sparked this whole question, and she told me a thought I hadn't thought about before: Maybe adulthood isn't a switch like I thought it was; maybe it's a collection of rights and responsibilities that society gradually transfers. This thought really was philosophically elegant, until I realized it was legally complicated to have to give to someone different rights at different times and track their legal “maturity.” 

So was 18 actually “the best guess”? 

Was 18 chosen because: 

A. humans objectively become adults at 18? 

Probably not. 

B. 18 is completely arbitrary? 

Probably too simplistic. 

C. societies needed a workable boundary and 18 became historically and internationally established? 

Maybe, yes. Because a practical boundary isn't necessarily a scientifically meaningful boundary. And that's an extremely important distinction because 18 can be useful without being biologically magical. 

Now that we know that 18 isn't a magical developmental threshold, we can recognize that there are legitimate practical reasons for fixed legal boundaries. The widespread adoption of 18 has historical and legal foundations rather than being purely biological, and different capacities already have different age thresholds. 

This means that 18 can be a reasonable legal boundary without being the moment at which every human being becomes fully adult. 

The question may not ultimately be: What is the correct age of adulthood? But rather:

Should adulthood be treated as an age at all? 

And so, I will leave this question for you, dear reader, to reflect upon!

Works Cited 

Best, John R., and Patricia H. Miller. “A Developmental Perspective on Executive Function.” Child Development, vol. 81, no. 6, 2010, pp. 1641–1660. 

Blakemore, Sarah-Jayne, and Suparna Choudhury. “Development of the Adolescent Brain: Implications for Executive Function and Social Cognition.” Journal of Child Psychology and Psychiatry, vol. 47, nos. 3–4, 2006, pp. 296–312. doi:10.1111/j.1469-7610.2006.01611.x. 

Cornell Law School Legal Information Institute. “Age of Majority.” Legal Information Institute. 

Crone, Eveline A., and Beatriz Luna. “The Role of Experience in Adolescent Cognitive Development: Integration of Executive, Memory, and Mesolimbic Systems.” Neuroscience & Biobehavioral Reviews, vol. 70, 2016, pp. 1–12. 

United Nations Children’s Fund (UNICEF). Convention on the Rights of the Child. United Nations, 1989. 

United Nations Children’s Fund (UNICEF) Innocenti Research Centre. Growing with Rights: A Guide to the Convention on the Rights of the Child.