I agree that he was being sarcastic about whether this was a realistic reason for him leaving. But I still think it's telling that he made the joke - it seems to me to indicate that there's some perception, internally, that teenagers no longer view them as cool.

For example, if I were to leave Google, and had the same sense of humor as Blake Ross, I might say something like "I'm leaving Google because my mom and her friends don't use Google+. But seriously..." I wouldn't actually leave Google for that reason, but it doesn't make it an untrue statement.