It's when you start recognising these tell tale signs:
You never miss the chance to use a loo. Just in case. Yes, you used to ask your children if they needed the loo before they left the house, and now you remind yourself to go. And in the pub, the supermarket and the station.
Saturdays in Top Shop are The Worst Place on Earth. You feel you have been beamed onto some alien planet. The noise, the music, the clothes, the people. You exchange sympathetic smiles with other exhausted mums.
Your parents start discussing their funerals with you, and telling you how much money is hidden under the mattress. You realise with dismay that your parents are not going to be here forever, and you remember all too clearly when they and their friends were the age you are now.
You walk into another room and wonder what you wanted. You needed something, but what exactly was it? The answer will come to you when you go back to the starting point, especially if it involves flights of stairs.
Muffin top is no longer just a phrase in Jamie's latest cook book. You can still get into your jeans, but there's always that annoying roll of flab hanging over the waistband, no matter how much you exercise. Tunics are great.
You are startled at how young the couples look in wedding photographs. Why are they marrying so young these days, you wonder? Then you read the small print and realise they are older than you were on your wedding day.
You discover it's not a freckle, it's an age spot. It slowly dawns on you that those annoying brown marks and your arms, legs and hands are not cute freckles. They are sun damage and you are slowly turning into your mother ( or father.) Brown Spot Remover goes onto the shopping list.
You sigh as you get into or out of a chair. Why is it a relief to sit down, and just a tad harder to get out of the low chair? You now realise why nursing homes have high backed wing chairs.
You are invited to a school reunion. This means there is a significant number of years between you and your O levels or A levels. You hunt for your school photographs, spend the night reminiscing about life in 4B, and join Friends Reunited. You don't see the point of Facebook any more.
You meet a friend you haven't seen for ages, and notice how old they look. But then they are probably thinking the same about you. You've always dismissed Botox as being for shallow people, but suddenly it seems rather tempting.
You find yourself using the same expressions as your parents. You're not quite sure how this happened, but you find yourself saying the same things to your kids that your parents said to you. Such as, "You'll understand when you have kids of your own," or even "Wait until you get to my age."
You suddenly notice how teenagers have such loud voices. There are only four of them in your train carriage, or in the cafe, but the volume of noise equates to at least 20 adults. Surely you were never like that?
You have to get your child to teach you how to use your new phone. And he's only 10. How has he got the technology gene and it missed you? Why can't they just keep things simple, you wonder.
You start enjoying the Antiques Roadshow. It's not so much that you are working out your inheritance, it' s just so well, interesting. All those nice stately homes and gardens.
A good Friday night is staying in and watching Gardeners' World. You snap at your kids, who are dashing off to yet another party or sleepover. "Be quiet, I'm missing how to pot up my begonias." And Monty Don....why is it only now that you realise he's quite nice, actually.
Men still whistle at you. But then you realise it's your 14-year-old daughter they are looking at. And you don't really know what to feel. Protective because they are lusting after her, proud that she's so gorgeous but admittedly, a teeny bit jealous.
You long for any early night. And that's no longer a euphemism. Not only have you become middle aged, you have turned into Ann Widdecombe, who suggested she preferred a long bath and a book to (mediocre) sex. The fact she has never had sex is beside the point, but sometimes it's all such a distant memory you wonder if you ever have.
What else would you add? Do you think middle-aged a state of mind or a physical age?