When you have the chilling realisation for the first time in your life that you were born 30 years after the end of WW2.
When you walk past someone handing out fliers for a trendy new club and they don't offer you one
When the lines around your eyes meet up with the lines around your mouth to make one giant set of lines running down your whole face
When you look down and see your mum's hands poking out of the ends of your sleeves
When you see a tv ad for 'Now 247' and you don't recognise a single song, then realise you have no idea what's in the top 40 (do they even have a top 40 on a Sunday afternoon now?) And when you do hum a modern song to yourself and some child looks at you with incredulity and asks you how you know it.
When a night out has to involve food, somewhere to sit and the music can't be too loud (oh yes, it turns out that there is such a thing as too loud). And if it's cold, you're wearing a coat. That ability you have to wear a thin little dress and not to feel the cold will vanish.
The hair, oh God, the hair! The hair on your head gets wiry and lots of hairstyles start to look ridiculous on you. Your eyebrows start to become invisible, your eyelashes get shorter. You start growing a beard to rival Brian Blessed's and your toes start to look like they belong to Bilbo Baggins.
You stop getting excited about new clothes and start getting excited about new household items.
You look at young women wearing fabulous high heels and instead of wondering where you got them, your first thoughts are about bunions and blisters.
But, on the plus side, you develop a level of confidence that comes with age. You know who you are and what you like. You know that the things that would have been the end of the world in your twenties actually really don't matter that much.