Teenagers are supposed to be the group most likely to lack manners, but I have noticed that they usually have better cell phone manners than most adults. Adults will routinely answer their phone in front of you, maybe saying "excuse me" or turning away, but not always. Teenagers, on the other hand, leap up and leave the room when their phones ring; this may be caused by an excessive teen need for privacy from adults, but I have seen them do it with their friends, too. Someone talking loudly to themselves in public is nearly always in their 30s-50s, too busy to take time out for the conversation, too self-centered to realize how they affect people around them or how much information they are divulging to strangers (I have heard amazing things, both business and private, in the grocery store aisles). Most teens I know would be mortified to do this; they lower their voices or find a quiet spot to talk. Teens are growing up with cell phones and creating appropriate etiquette as they go; they are still sorting out how to be part of a group, so they recognize how their behavior affects their friends and adjust it. Adults, on the other hand, are all too often past learning new behavior.
Maybe one reasons teenagers like text-messaging so much is that it is quiet (and private).