Not ONLY teenagers but adults watch anime or read manga as well. Simply, it depend on a person. Some watch it by himself/herself, some with his/her children, others don't. There are very popular anime like "Totoro" from The Studio Ghibli Company, and everyone likes it as far as I know. About manga, adults also like to read it but watching anime is more popular, I guess. It's because we don't read manga with children (I'm not talking about kid's books), and the manga companies don't play commercial on TV.