I've seen grown adults behave this way with their phones. Totally zonked out and unaware of anything happening in the real world. Don't think it's an age thing.
I have a weird quirk that when talking to someone, if they start using their phone, I stop talking until they're done. It has made me painfully aware how true your comment is. Many times people don't even notice that I stopped talking (and why event try to talk to someone that isn't listening?)
I don't know how society managed to normalize playing with your phone during a face to face conversation. Like, imagine going back to 1990 and getting into a conversation with your friend over coffee, and in the middle of him talking, you whip out a newspaper and start reading it. He would instantly say "WTF are you doing, man?" But today, the same thing happens with the phone, and it's just kind of...accepted?