My therapist said that Endogenic systems are perfectly possible, in psychological context.
She said that, as far as psychologists can tell, all people are born with a fragmented brain, and that there's no guarantee that your brain will 'solidify' into a single identity when you're nine-years-old.
She says that trauma does not have to be a cause for systemhood, and that age does not have to play a part either; even if it commonly does play a part, it does not have to.
She also says that psychologists and scientists spend too much time focusing on the incorrect idea that you can't become a system unless youre a child with trauma, and that a lot of psychologists are so focused on the current knowledge, that they don't look to expand their knowledge and discover more about mental diversity.
So yeah, just wanted to share. :)