Why are majority of teenagers lacking in basic grammar?

[Madeline]

I see it on social media, like Facebook, and much on Yahoo Answers. They spell most of what they write wrong, and forget to use punctuation. They go to school though, because half the questions mention someone or something related to highschool. Are they really grammatically impaired, or are they just to lazy to type properly on their computer? I may not have the most mature voacabulary as a teenager myself, but at least I make an effort. Teenagers, why do you not type properly?