Well, it's when you're walking down the street - and you look great from behind. A passing motor car (usually full of youffs - and probably an souped-up pseudo sports jobby) pips its hooter. Clearly the driver thinks that you're 16.
It's only when they have driven past you - they turn around to cop a better look and sadly realise that you're closer to 61.
That's a 1661 moment.
Welcome to the world of the middle aged woman :)
(actually - it's never happened to me - although I did get the thumbs up from some young men last year whilst driving around a roundabout...the fact that I was wearing huge sunglasses and therefore they couldn't see my wrinkles probably had a lot to do with it...)