Well, earlier today I grabbed my sisters Ouran Host Club manga and began ready all of Renge's lines in a loud, obnoxious (Yet still able to grasp the emotion in all of her rants) voice. My second most viewed video on youtube is about poop. My most viewed video on youtube (Though I was not the one doing the voice) is a Persona -Trinity Soul- dub, in which Akihiko repeatedly yells "Yaoi". I occasionally speak to my sister in a loud British or Irish accent. And I frequently use terms like LOL, OMG, and IDK in real life.
Also, I laugh at the fact that my initials are JK.
I'd say that's sufficient childishness to say my mental age is about 6.