Remember Eleven? Don't look at me like that. That was a rhetorical question. Hmm...so the thing about Eleven was that he always felt like an old soul trapped in a young body. His wise moments, his speech in The Rings of Akhaten and his general viewpoint all reflected that however young and immature he appeared to be, he still was an old being with oodles of experience and history. And then we have Twelve who is err....how do I put it? Well he just feels like the polar opposite. You know a stubborn 12 year old in an older body. "What makes you say that?" you might ask. Well let me explain.
"So you've got a whole room for not being awake in. But what's the point? You're just missing the room."
Spoken like a child, the curiosity, the wonder and the trying to make sense of the odd world around him. Sure you could say that was post regeneration trauma messing with his head but I like to think it as a pointer to his overall fresh persona. Then we have Into the Dalek with Clara basically mentoring him about what is right and what is wrong like a teacher mentors a student. Oh and how could we forget about that slap (though I am totally against hitting kids), it feels like a slap a child would get for being extremely impertinent.
Moving on to Robots of Sherwood where we see the childishly competitive side of Twelve. His constant quarrels and bickering with Robin Hood only add to his childish escapades and force Clara to act as, dare I say, a babysitter. Remember the "Will you to shut up?" scolding that Clara gave in the dungeon? Watch it and tell me I am wrong. We have a similar scenario with Doctor and Santa in Last Christmas. Here again we see Clara step up from being a companion to a babysitter with "Doctor, behave!". Also Twelve has that innate sense of stubbornness which is characteristic of a childhood. Children often feel that they know better and that all others are basically idiots not keeping up or pudding-brains as Twelve might say. This reminds me that Twelve also has the childish nature of giving people nick-names and hilarious ones at that (Which one’s your favorite?).
In the Forest of the Night is basically where he goes full throttle. His interaction with the kids, his mannerisms and even his deductions make him seem like a child. I know it's a cute story but I saw that twist com-...Oops digressing again. As I was saying, remember how he just pops out of that tree when Ruby asks the question about the trees having no rings or his totally absurd idea about looking big like a three headed, six legged scary thing to scare off the wolves or just that scene where he goes through a whole row of kids looking for Maebh (Oh that one is pure gold). They all portray his inner child to the tee.
Actually if you examine the whole of series 8 for that matter you'll find hints of his childlike behavior scattered throughout. Things that you wouldn't expect from a 2000 year old Timelord whether it is giving away his favorite watch for a stinky coat or turning Clara into his carer or fighting with Danny or forgetting faces and deleting names or just his constant yammering but actually not saying much. Children often speak a lot and a lot of it is nonsense but they actually say very little about how they really feel. This is one thing that I think fits perfectly into Twelve's personality. He is always on about crazy planets and fantastic places and eras but he seldom expresses how he feels unless of course you are irritating him, then it's all Shut-Ups or Hitting-with-shoes.
But this is just one facet of his character (There’s also that magician angle. I mean what’s that about?) ….and there is still plenty to dissect and dismantle from Series 8.