"Give me a child until he is seven and I will show you the man," said Aristotle. Since ancient times, there has been something special about this age. Once children are seven, they are considered by the Catholic Church to have reached the "age of reason". Until 1933, seven was also the age of criminal responsibility in the UK. In 1964, filmmaker Michael Apted decided to test Aristotle's premise: interviewing a group of 7-year-old British children from a range of backgrounds about a range of subjects for ITV documentary, Seven Up! (and revisiting them every seven years after that).
But what is so significant about this stage of childhood, particularly when it comes to boys? As Prince George turns a magnificent 7 today, the question will be one his parents, the Duke and Duchess of Cambridge, may well have on their minds.
As the parent of a boy who turns 7 himself in the next few months, I too am beginning to wonder: how much of my son's character will be established by then, and what kind of parenting does a 7-year-old boy really need?
The best place to start is the science: around the age of 7, important changes take place in the human brain. The frontal and temporal lobes, which are responsible for our cognitive functions, grow tremendously. At the same time, by around the age of 7, children become more capable of handling their emotions. At school in Britain, children of 7 enter key stage two, as their learning abilities progress.
"One of the things that happens in terms of cognitive development is, before the age of 7, children tend to be very categorical," says Anita Cleare, parenting expert and author of The Work/Parent Switch. "It's black and white, there are 'goodies' and 'baddies'. Once they get to about 7, they are capable of more nuanced thinking. It's a real step forward."
It is also around this age that children begin to develop a sense of self. "They start to [consider] 'what kind of person am I and what things do I like?'" says Cleare. "It doesn't mean they stick with those preferences, but they start to explore those areas of identity."
Their sense of justice and moral understanding develops too, in tandem with their sense of humour, and conversation with children of this age becomes a little more sophisticated. They begin to understand that words can have more than one meaning. There are fewer scatological jokes (if you're lucky).
Meanwhile, they are becoming social creatures by now, learning the kind of civility and empathy that so often seems to elude children aged 6 and under. "There's a shift towards friendship and what it means; [towards building] real attachments to other people, and there's therefore also that potential to get hurt when attachments break," says Cleare.
She believes differences between boys and girls at this age are more likely the result of nurture than nature, but that boys do tend to become especially active and less inclined than girls to sit still and do craft activities.
In his bestselling book Raising Boys, parenting expert Steve Biddulph suggests the second stage of a boy's development starts around the age of 6 and lasts until 14. This, he says, is "when the boy, out of his own internal drives, starts wanting to learn to be a man and looks more and more to his father for interest and activity (although his mother remains very involved, and the wider world is beckoning, too.)" It is the age when a boy "becomes happy and secure about being male." His masculinity seems to be "switch[ed] on" at this stage.
But for all the rough and tumble, and increase in traditionally "masculine" behaviours or attitudes from 6 or 7 onwards, parents should be wary of neglecting to nurture their son's emotional side, experts caution. "There's lots of evidence we don't have as many conversations about emotions and feelings with boys as with girls, and that has an impact in terms of academic achievement and emotional development," warns Cleare.
One way to put this right, she advises, is to discuss the feelings and motivations of characters in the books we read with our sons at this age. Getting them used to talking about and understanding feelings is crucial as they begin to model themselves more on the key male figures in their lives. Biddulph, likewise, recommends hugging our boys, talking to them and listening to their feelings.
For a parent it can be poignant watching your son leave his baby years behind and begin to show a new maturity from around age 7. But those who have been through this stage agree that it's also incredibly rewarding.
In the pictures released to mark his momentous seventh birthday, Prince George grins boldly at his mother's camera, his new adult teeth on display. He retains some of his former cherubic appearance; but looking straight into the lens, there is something more composed about him now: the sign, perhaps, of a boy who's ready to take on the world, with a smile.