He's always had stacks of personality, but the latest picture of Prince Louis, taken on his seventh birthday, is a real corker.

He bears a striking resemblance to Grandpa Middleton; but there's something about him that also reminds me of his great-grandfather on his father's side, the late Duke of Edinburgh.

He has the same mischief dancing behind the eyes, the same aura of boundless energy. 

'Give me a child until he is seven and I will show you the man,' wrote Prince Philip's countryman, Aristotle. You can already see the outline of the young man Louis will become.

But no need to rush. For now, he's still a little boy. He may look grown-up for his age, dressed as his dad's mini-me in a checked shirt and scratchy green jumper, but that gappy smile gives it away.

Entirely unselfconscious as he gazes boldly into the camera, you get the sense that this is a child who has known only love and happiness. 

I remember when my two were the same age: there was a simplicity to their existence that, looking back, seems so precious.

At seven, real life, with its challenges, is still very much at arm's length, and everything is so much better for it. 

Children that age are joyous, unfiltered, unencumbered by the anxieties and disappointments of the adult world.

When my son was seven, for example, he still entertained the notion that he would one day play for his beloved Chelsea. And why not? Childhood is short: no need to rush reality.

Children are life-affirming at all ages – but for me as a parent this was one of the most rewarding. 

They're still small enough to be relatively biddable, but they're old enough to have ideas and opinions of their own.

They still possess a sense of wonder about the world – and yet they are not helpless in it. 

You can have proper conversations with them, but they still remind you of the delight that's to be found in sheer nonsense.

I understand completely why the Princess of Wales is determined to spend as much time with her children as possible, especially in the context of her recent illness. 

Such joyful innocence can never be preserved, of course – but if Louis is to be her last, she will want to be by his side every step of the way as he begins to cut his (new front) teeth in the big bad world.