If you're someone who likes concrete answers, then Legion probably isn't the show for you. Rather than making more sense as it goes along, each new episode has only deepened the sense of confusion and raised more mysteries to be solved. Thankfully, the relationship between Cary and Kerry on Legion doesn't need to be one of those lingering questions like what exactly the Devil With The Yellow Eyes is and whether or not everyone on the show is actually trapped inside David's mind. But the show has been fairly — and unusually — explicit when addressing the powers of the two Loudermilks, including solving the puzzle of why Kerry is so much younger than Cary.
In "Chapter 4," Legion explained the mechanics of Cary and Kerry's powers; when Cary was eight, he realized that his body was also home to a second — and entirely separate — person, a young native girl named Kerry. While he's the brains of the operation (and also in charge of the boring stuff like eating, sleeping, and going to the bathroom), she's the brawn; Kerry pretty much only leaves Cary's body to do battle, and returns to him to recharge afterwards.
So if they were born at the same time and inhabit the same body, why is one of them so much older than the other? At 66, Bill Irwin (who plays Cary) is a whole four decades older than 26-year-old Amber Midthunder (who plays Kerry). What accounts for the discrepancy? Since the Loudermilks are original creations thought up by showrunner Noah Hawley (Fargo) rather than characters pulled from the pages of Marvel comics, there's nothing to go on when it comes to the source material.
Fortunately, the age differential between the Loudermilks was addressed on the show itself when Cary explained that Kerry only ages when she is outside of his body. So while he has been aging at a normal human rate, she has only grown during the intervals in which she separates herself from his body. In fact, given this explanation and the fact that she seemingly only leaves Cary's body to train and go on missions, it's actually sort of impressive that she's managed to age all the way to 26.
The very strange relationship between Cary and Kerry is what attracted Irwin to the role in the first place. "Noah and I sat down, we ordered breakfast, and he started talking about a character who, to his surprise, had another being inside of him," the actor told The Hollywood Reporter in a recent interview. "He didn't mention FX. He didn't mention Marvel. He didn't mention X-Men. He went straight to this notion of a guy my age who shared his body with a young gorgeous warrior of another gender. That was our breakfast conversation."
According to Irwin, the generational gap between Cary and Kerry makes for a very complicated, as it's neither paternal nor fraternal, but rather something in between. "It's father-daughter, but it's also brother-sister," he told THR about their unique but inextricable bond. "We need to be locked together inside of each other for sustenance to happen."
Irwin also drew attention to the fact that all of the powers Hawley has chosen to highlight on the show — both ones he conceived of and ones pulled from the comic books — share a theme of identity: including David's fractured psyche, Syd's body-swapping, and Ptonomy's memory-mapping. He explained:
Cary and Kerry may not have the most conventional relationship (or the most comprehensible one) but it's clear that the identity and dependency and empathy that the Loudermilks share — despite the drastic differences in their outward appearances — are key to the themes of Legion.