Fashion is whatever is in vogue at a particular time and place. It may be clothing or it may be interior design or it may be the current hairstyles.
We have certain things that are in fashion today but we can look back to, say 1900, and show what was in fashion at that time. Gibson Girl hairstyles for women for example - skirts that fall to the ankles and high necked blouses.
You don't have to be out of fashion no matter how you dress - just choose what fashion you want for yourself and do it.