So I remember seeing someone on Twitter theorize that the Saja Boys' names (minus Jinu) are based on their past lives and how they became demons, so here's my take on Baby's cuz I suddenly got attached to him for literally no reason other than that he exists, I guess
So most people would probably think Baby made a deal with Gwi-Ma to stay youthful or whatever, but I like to think that the reason he looks so youthful was because he FELT childish when he made a deal with Gwi-Ma, especially when it comes to WHY he took it in the first place, which brings us to my next headcanon.
I personally hc that Baby had a little sister who was better than him in everything he's good at, taking all the attention from their parents and everyone around them towards her, and making him grow bitter and resentful as he got older while she got more popular within the community. It eventually got to the point where Gwi-Ma sensed his jealousy and took advantage of his insecurities to strike a deal: He can be the center of attention once more... In exchange for his sister's soul.
Baby refuses at first, but eventually takes the deal when Gwi-Ma continues to taunt and fill his head with self-deprecating comments, leading him to isolate his sister in a nearby woods where her soul will be taken by one of Gwi-Ma's henchmen.
With her gone, Baby was able to build his reputation back up again, but now had to live with the guilt and shame of getting his sister killed in order to do so, and after years of self-torment, he decides to run away from his community, patterns soon overtaking his body and turning him into the demon we all know and love.
So in short, our youthful-looking Saja Boy made a deal with the devil because he wanted attention, just like a child... Or a baby, if you will.