I've started to wonder if something specific happened in Jake's life at age 13-15, somewhere in that vicinity.
There's this plotline in 30 Rock about a teacher (female) who commits statutory rape with a student (male), both believing themselves to be in love. Decades later, when the teacher gets out of prison, she finds that the student has become an adult living in a permanent state of adolescence, obsessed with comics and superheroes, unable to form meaningful adult relationships.
Something arrested Jake's development in a very similar way, and it shows. The "I've never masturbated, don't even know what that stuff looks like" lie. The sneering at alcohol, or adult ways of having fun. The childish interests he can't move on from (even as the cousins he played with move on to more grownup lives). The avatar of a young child being used to represent his secret, true self. The paranoia that if you don't want to have sex with anyone at all, people will try to coerce you into realizing that you're a sexual being by forcing you to copulate (even if you almost never leave the house). The way that "teenager" in his tweets is almost always accompanied by the words "stalk" or "groom" or other sexualized, predatory terms.
I wonder if one of Jake's aunts or teachers touched him in a no-no place.