A couple of weeks ago my other half took her cousin and his wife (in their early 20s) to JFK for a short domestic flight leaving at 6am. She was wearing 5" heels and he was dressed to the nines. Much to our amusement we later learned from another family member that they like to dress up because other passengers might think they are someone important or a celebrity. When we learned this we struggled to keep a straight face
You'll find me in a polo shirt or maybe button up shirt if I've come from the office wearing either jeans or shorts depending on the temperature. On longer flights I'll change into t-shirt and PJ bottoms once onboard, unless the airline provides them.