David Bainbridge's children are not yet teenagers, which may explain why he is so excited about what happens to human beings between the ages of 13 and 20. To him, adolescence is not just a painful stage of rows, door-slamming, self- obsession and rooms knee-deep in dirty clothes, it's what sets us apart from other species.
But then he's a zoologist. His profession gives him a different angle on the years between childhood and adulthood. Drawing on evolutionary biology, palaeoanthropology, psychology, his own happy memories of being a Led Zeppelin fan, and a chirpy prose style, he examines the teenage years with reverence, not to say awe.
Somewhere between 800,000 and 300,000 years ago, the teenager emerged (grunting, no doubt) from his cave. From dental remains, it can be calculated that, during that time, the age of maturity crept up from ten to its current level of 18 for girls and 21 for boys. Other mammals of similar size mature much more quickly; they are reproducing by the age of three. If human beings hang around asking for money and raiding the fridge then, as a scientist, he believes it must be for some better reason than to annoy their parents.
The brain reaches its maximum size at the age of six, but there is plenty going on under those elaborate teen hairstyles. The lengthy human growth phase provides a sheltered environment in which the frontal cortex, the thinking bit of the brain, can be switched on, allowed to make novel, creative connections, and then pruned for added efficiency. And all this happens while teenagers are still vulnerable enough for parents to want to protect them.
Bainbridge runs through the physical changes of those years - spots, sweat, grease and growth spurts - and explains the evolutionary advantages they confer, with varying conviction. Pubic hairs, for example, may exist as wicks for sexual scents, or merely to prevent chafing. He cannot find an argument for acne, which is just an unfortunate, usually short-lived, hormonal mistake.
The changes in the brain interest him more. They make the difference between Homo erectus - a dull creature - and Homo sapiens, evolution's masterpiece, capable of adapting to changing circumstances with unique ease. All the worrying sides of adolescence, he argues, are the reason why we are the planet's dominant species. Self-analysis makes teenagers moody and intense, but it leads to new ways of doing things. Risk-taking may be nerve-racking for those who are nearest and dearest, but it does develop one's confidence.
The downside of the super-sophisticated teenage brain is that it is vulnerable to mental illness - depression, schizophrenia and anxiety are unique to humans. Onset is generally between the ages of 13 and 18, and it's the price we pay for our brains being almost too complicated for our own good. As adults, however, we shouldn't take a determinist view, because all these disorders develop more frequently in adverse circumstances, and are therefore less likely to occur in teenagers who have strong adult support.
Much as it pains the author to say it, having enjoyed wild times himself, his research leads him to the conclusion that, for the teenager, drugs of all kinds - cigarettes and alcohol, as well as the usual class A, B and C ones - are playing with fire. Each has its own menu of effects, but they all work on the dopamine receptors in the brain, making them less receptive to other non-toxic, non-addictive sources of pleasure, such as a hearty walk. Reluctantly, Bainbridge admits to having an "epiphany" in which he realised that "the old advice that 'these things are best left till college' might turn out to be correct".
Teenagers is an entertaining book full of quotable "Did you know . . . ?" facts or assertions. DYK, for example, that the age of puberty decreased by 12 days for every year of the past century? Or that hormones don't trigger an interest in sex; that's the job of GnRH neurons? Or that, at any one time, 50 to 75 per cent of teenagers describe themselves as depressed?
But he has a message, too. To those grumpy adults who ask, "What are teenagers for?" he suggests the real question should be: "What are adults for?" Our brains are inflexible and declining; our creativity is limited. Rather than feel sorry for ourselves when teenagers snarl or make a nuisance of themselves on the bus, adults should take a humbler, more positive line. Teenagers are our only excuse for living longer. Without them, without their need to be guided, paid for and mopped up after while their brains mature, we would all be dead by 30.
Remember this the next time the door slams, or the kitchen is left looking like a bomb site. I wonder if the author will remember it himself when his own children are a little older?