I note that Polmont is a Male YOI, this is pure speculation but I wonder if this claiming to be an adult baby is in response to not being moved to a girls' YOI upon starting to identify as female.
Apologies if I've got the timeline wrong. I really don't want to re-read that. The young man who was murdered didn't deserve to be killed, whatever his crime.
I also note that being a baby, needing a dummy etc doesn't extend to this individual not being able to express their needs in a very clear, adult manner. Feels like manipulating the system to me.