Why Do People Think 40 Is Old?

It's always been a question of mine. Why do people think that their 40th birthday makes them old? Something I've seen on a few occasions are these signs that are taped to the lightposts in my neighborhood that usually include the words "Look who's turning forty!" followed by a picture of the person and their name. I once asked my mother why somebody would announce their birthday to everybody who drove by. She answered me that when you turned 40, you were often considered old, so people would play jokes on those of their friends and family who had their 40th birthday coming up. I was confused. Why would people go to the trouble of filling somebody's front yard with signs at 6:00 in the morning (another phenomena I've witnessed; the people spent hours making black "happy 40th birthday" signs then spent another hour "decorating" their friend's yard) just to announce the birthday of a person who was turning forty? After all, 40 years is less than half of your life. The average female lifespan is 82.8 years. As a male, you're a bit less lucky, with an average lifespan of 79.2 years. So as a male, 40 is a little bit more than half your life, but as a female, you still have 43 years to go. So really, 40 years is not especially old. People sometimes begin feeling old when they turn 40, but this is because 40 was old in earlier days of humans. It was natural for you to die at around 40, due to disease or organ failure or other natural causes. It was just how people were. Anybody living longer than 47 or so was considered pretty ancient. But with modern medical technology, we are able to extend these lifespans so they are double what they were in the 16th century. This change occured somewhere during the end of the 1700s and the 1800s. So 40 is no longer very old, considering how long you have left.