Why do people worry about age so much?

And why are some people quick to say that someone who is 25 is almost 30, or 35 is almost 40? In my opinion, 39 with a birthday coming in a month or two is almsot 40...same with 29 with a birthday coming soon is almost 30. I am also tired of people in their 20s and 30s saying that they are OLD...whatever. They are stupid methinks. Also, why is it that as a person in my 30s, I act younger than some people in their 20s, and they act like I should act so old and mature. I actually embared a 26 year old coworker of mine, because I did a little happy dance in a shop when I got a free folder of a Korean star I adore. She said it is embarasing to be with someone who is almsot 40 acting like that. I don't think I am almost 40 either. I still have a few years to go...geez. So at 26 is she almost 30? Why do people in their 20s/30s act so darn serious? As in OLD?