Stroppy, smelly and surgically attached to their smartphones... it's fair to say teenagers have a pretty poor reputation.

And, when it comes to understanding them, they might as well be creatures from a different planet - as any parent with adolescent children knows.

But new research has revealed that one of the worst teenage traits - that lingering scent of stale sweat - may not be their fault, reports the Daily Mail.

Researchers from Aarhus University in Denmark found that teenagers aren't just ignoring the pungent aroma that so often surrounds them; in fact, they can't even smell it.

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The study, which tested common odours on more than 400 participants, showed that one in seven youngsters aged between 13 and 18 can't recognise the smell of sweat.

While it's bad news for parents, the findings shed new light on the mysteries of teenage development - from why they lie in bed until midday to how often they think about sex.
We reveal everything you never knew about your teenager...

Bodies grow "outside-in"

Teens grow at an alarming rate - boys by up to 9cm a year and girls by as much as 8cm - requiring an almost constant supply of new shoes and school uniforms. This phenomenal growth spurt starts at the outside of the body and moves inwards. Hands and feet are the first to expand, followed by the shin bones, thighs, forearms and upper arms.

Growth is triggered by increased levels of the sex hormone testosterone. Girls do most of their growing between the ages of 12 and 13 and boys between 14 and 15.

Most teenagers shoot up so fast that their brains can't keep up. As they get taller, their centre of gravity lifts, but the brain cannot calculate how to balance its new frame, making the body gangly and awkward.

They don't see danger

Teens brains are not fully developed leading them to be bigger risk takers . Photo / Getty
Teens brains are not fully developed leading them to be bigger risk takers . Photo / Getty

Driving too fast, skiving off school, trying illegal substances at parties... what seems like reckless risk-taking to you and me is just a day in the life of an average teenager.

But scientists have shown that, far from being stupid or wilfully ignoring the risks, teens don't actually see danger because of the way their brains are wired.

The human brain isn't fully formed until long after adolescence (roughly age 20 in girls, 24 in boys).

While it's undergoing these changes, however, the workings of the pre-frontal cortex are disrupted. Parenting expert Anita Cleare explains: "The regions associated with planning and decision-making are finished last, which means the part of the brain used to weigh up risks and make balanced decisions is only half-built."

Sex on the brain

Parents of teenage boys may be surprised it's not more frequent, but adolescents of both genders have been shown to think about sex up to every six seconds.

That's 600 times an hour, or approximately 9,000 times in a waking day.

This obsession with sex is sparked by hormones: the teenage brain is flooded with oestrogen and testosterone, released at a rate of 50 times more than in the pre-puberty years. These hormones can cause teenage mood swings, by upsetting the chemical balance of the brain.

Girls use twice as many words as boys

On average, girls use twice as many words as boys. Photo / Getty
On average, girls use twice as many words as boys. Photo / Getty

The stereotype of a chatty, eloquent teenage girl and a gruff, grunting adolescent boy is surprisingly close to the truth. Girls' brains are inbuilt with a head-start for language: they tend to talk earlier, have larger pre-school vocabularies and use more complex sentences.

This is all to do with the structure of the brain. In girls the "Broca" area of the brain, where we produce language, contains 20 per cent more neurons than the male brain, and the "Wernicke's" area, where we interpret language, is 18 per cent bigger in females. Girls also use both sides of their brain while reading, writing and speaking, while boys only use the left.

Teens need as much sleep as toddlers

If your teenager regularly sleeps until noon, or struggles to get up for school, don't assume it's laziness. At an age when their brains and bodies are growing so quickly, teenagers need between nine and ten hours sleep - as much as a young child.

Teenagers who sleep less than 7.5 hours a night are technically sleep-deprived, while a study by Brown University in the U.S. showed that those who get nine hours' sleep are more likely to achieve higher grades.

Their brains shut down when you nag

What you always suspected is true; teens brains shut down when you nag them. Photo / Getty
What you always suspected is true; teens brains shut down when you nag them. Photo / Getty

It may seem like you're speaking to a brick wall, but don't assume your teenager is ignoring you. Studies have shown their brains actually shut down when faced with criticism or rebuke.

Scientists at Harvard University monitored 32 teenagers in a brain scanner while they listened to short clips of their mother's nagging. They found that the areas of the brain that process negative emotion were alerted, while the areas that control emotion and allow us to see things from another's point of view were deactivated entirely.

What's more, teenagers have poor prospective memories, meaning they aren't very good at holding things in their head. "When you nag them, it really does go in one ear and out the other," says Anita.

They're blind to others' emotions

There's only one person who matters in a teenager's world: themselves. This narcissistic behaviour isn't sheer self-centredness; it's because they struggle to recognise emotions in others.

Studies have shown that adolescents are 20 per cent less accurate in gauging the emotions depicted on people's faces, not fully grasping the ability to detect fear, shock or anger until age 18.

This is because teens rely on their limbic system - the more primitive back part of the brain - rather than the pre-frontal cortex, which is still developing, in their interactions with others.

Solving sudokus faster than you

The can solve sudokus faster than you. Photo / Getty
The can solve sudokus faster than you. Photo / Getty

Though solving sudokus and doing crosswords is unlikely to fit their idea of fun, teenagers are surprisingly skilled at logical puzzles. Studies in America have shown that adolescents adapt to changes in rules more quickly than their adult counterparts, making them good problem-solvers.

Why they're more prone to addiction

The immature teenage brain is more vulnerable to addictive behaviour than the adult brain, so drinking, smoking or taking drugs can be disastrous.

Young people who drink alcohol are 50 times more likely to take cocaine than teetotal teens.

Just as learning a fact is more efficient in the adolescent brain, so too is addiction. Putting a substance into the body stimulates synapses, which then build a "reward circuit" around that substance, encouraging the body to crave it even more.

"When teens get something they want, their brains release more dopamine - the feel-good chemical - than an adult's,"explains Anita. "This means their highs are higher. There's a huge neurological pay-off which can drive teens to repeat behaviours."

First love that has lasting impression

First loves make a big impact. Photo / Getty
First loves make a big impact. Photo / Getty

When your teenager gets dumped by their first love, it can seem like a tragedy of Shakespearean proportions.

But all that sobbing into pillows is far from melodrama; experts have shown adolescents to be capable of forming genuinely meaningful relationships.

Scientists say teenagers develop their own sense of identity through relationships with others, making early boy/girlfriends particularly important.

In a recent survey of 1,600 adults, a quarter said they would happily return to their first love and marry them.

Dr Diana Birch, medical director of the charity Youth Support, who has done extensive research on young relationships, says she has seen many stand the test of time.

"As teenage brains are primed for learning, we make more vivid memories during adolescence and these last longer," adds Anita. "Our first love can feel like the richest and most enduring of our lives."