Babies make you look young. New life, fertility, spring lambs. Children under 12 make you look amazing, because it's astonishing that anyone can keep up the pace. But once you have teenagers, you start to look like half an orange with the juice squeezed out.
This is partly because you never get any sleep. You're either lying awake worrying, or you're rudely awoken at 3am by the sound of the front door slamming shut.
But it's also because your slightly used appearance is bang next to the vibrant, shining beauty of youth. It's like having a before and after shot permanently side by side. Or putting a withered apple core next to a Cox's Orange Pippin.
'You can use my mascara if you like,' says my 15-year-old daughter, kindly. 'Thank you, darling,' I say, absently, thinking I'm probably more in need of masonry paint.
Time passes. Of course it does. And I wouldn't, for a minute, want to miss watching my babies grow up. But as they grow older, so do I.
'You have to realise,' said my eldest son the other day, 'that I'm 19.'
This was part of a conversation about what is a reasonable amount of beer for any one person to drink in a single night, so he wasn't actually making a point about how teenagers, by their very presence, make you feel like a wizened crone. But it stopped me in my tracks. I thought, how is this possible? Nineteen? I only have to close my eyes and he's there in my hands, 7lb 13oz, frowning against the light.
And then you think, if nineteen years passed that quickly, is time actually speeding up? Will I find myself, slightly dazed, with grey hair, bifocals and a pension the day after tomorrow?
Maybe the answer is to stop cleaning all the mirrors in the house. Reality might be more bearable with a kind film of dust.