I've recently thought about how these words are used. 'Swag' seems to have replaced words that describe something's coolness or fashion appeal. YOLO is a term used to say, "I am going to take a risk and have fun doing it." Sometimes, however, these words are used mindlessly.
But what does this have to do with anything regarding proper English? Everything, actually. Eventually, these teenagers are going to grow up and have children. Those children will have children. Perhaps these slang terms will take root and begin to replace certain words in our vocabulary.
Let's go back to the term 'Swag'. It is used in place of 'cool' sometimes. Many words also have done the same throughout our history, such as groovy, neat, far out, radical, you name it. However, cool was also used in the same context for just as long as these words were used. In the '10 era, not many words have survived and served the same purpose as cool. Perhaps 'swag' will be another word used in junction with cool.
When you take a moment to stop and think about our youth using new terms, you may think that they're being ridiculous and odd, but they are actually creating new words for our language and recycling old ones, whether or not they know it.