The issue for me is their ability to be fooled. If you want to make a point of sayin their mental age is 25, their mental age is 25. Also abuse literally ages children in terms of trust and suspicion so that sucks as an argument- it is genuinely used as a warning sign for possible abuse, children acting inappropriately for their age.
The part where they take this at face value and never cross reference against a memory where theycshould maybe not immediately believe this total stranger implies that at no point in their memories is there a scenario where you would need to examine someone's motives or verify a truth which seems unlikely given 18 years of past memories and 5 years of abuse. Abuse will teach you not to trust very quickly. Its honestly just frustrating.
Also with the "hormones and mental age" argument- listen in real life there is no such thing as being reborn with past memories. In real life your brain is constantly physically changing and building connections as you learn so to begin with the neuroscience of all this is suspect. If we are suspending our belief to say okay, they have these fully formed connections, it doesn't make very much sense to then say "well children's brains aren't fully developed" but then they have fully developed personality and memories and a supernatural psychic bond? I think we have to set neuroscience to the side here.