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But yeah, basically, Five should pretty much mentally still be a child with some severe issues of maladaptation and mental health problems like woah. He was thirteen and he was thrown into the apocalypse, with no other human beings!, with no source of food or nutrients!, and so on. He would probably have some pretty massive PTSD, not to mention extreme and long-term malnutrition that could lead to paranoia and hallucinations among other things. The idea that he'd become a Totally Normal Grumpy Old Man is just... unrealistic in the extreme.

At best, he'd have the usual run of PTSD symptoms, trouble communicating with other people, paranoia, hoarding issues, overreaction to some "normal" stimuli and total lack of reaction to other stimuli (for example, he might overreact to the sound of a car engine backfiring but not react at all to, say, cutting himself), and so on. And that's like.. the best case. He'd need help identifying what is and isn't okay to eat, and would be completely indiscriminate in terms of taste - getting him to "like" anything enough to prefer it would be a feat. Bathing regularly would be a struggle. His entire sense of what is and is not normal would be rewritten - unlike in the show where he seems to remember and be familiar with pretty much all aspects of modern society, he would probably have forgotten 90% of that because he was only thirteen when he left it behind and has spent the bulk of his life in the apocalypse. His actual maturity level would probably still be close to thirteen years old, because he didn't get a chance to actually mature like a normal person, had no models for maturity, etc. Let's not talk about how touch-starved he must be, basically having gone most of his life without being touched at all by anyone.

The shit with Temps would not have helped any of this. Sure, they recruited him or whatever, but do you really think they cared about his mental health or maturity level? They just cared about his capacity to survive, to kill, to time travel. (And speaking of, they likely would have had to train him to kill. Where would he learn to kill in an apocalypse with NO PEOPLE? Think this through, showrunners.)

And that's "at best". Which is probably not how things turn out.

Suffice to say there's no way in Hell that boy is coming out of this whole thing a composed, calm badass with an I Always Know Best attitude problem.

(Which is not to say I want to change the character completely. Five is extremely, blisteringly intelligent and I'm sure he fronts confident very well, especially after the Temps Agency taught him how to kill so very efficiently. But that's all it is: A front. A wall to hide the scared, deprived child behind.)