Body language is supposedly the universal language, building bridges between continents, but within our own society body language is what often separates generations. The transition of the age gaps has many changes and often those changes affect our body language. The way you sit, the way you stand, the way you dance, the way you act in your day to day lives signifies the type of person you are and often what generation you belong to.
The most drastic changes seem to be between parents and their children. As we grow we learn based upon our experiences, and most often our parents directly affect the way we respond to situations. Yet our body language is often in direct contrast to our parents as we fight to break away from their grasp. As we try to separate our selves we often are trying to fit in with a certain crowd and as we develop friends we evolve into a group that habitually acts somewhat alike. Although we may have very different characteristics and attitudes, the style of dress and body language directly coincides.
As you grow older you realize your self-confidence and your posture change to signify that change. Sitting up straight shows assertion for who you are and what you do. In the adult world to be successful you need to show an image of success. Once an adult you are sure of who you are and no longer need to fit into that "certain" crowd, and sitting up is a very subtle way of letting others know that. Plus, it's a lot better for your body, and often times those physical pains are more abundant as we age. Teenagers most often sit slouched back to show a relaxed "I"m cool" posterior, directly going against what their parents say. In the long run it's going to mess up our backs but for our age group it insinuates that we fit in. .
The way people behave around their peers is a very dramatic action in the body language. Adults often are much more professional, even in the company of best friends they have grown out of most of the playful movements teenagers do on a regular basis.