A female character with a mindset of 5 year old in a romantic setting with 30 year old man .. this dynamic itself introduces several questions like consent, power dynamics & exploitation.. a 5 year old can't consent to romantic & sexual relationships ..
Because the character has a 5 year old mindset of a 5 year old, any romantic pursuit willbe highly manipulative, & can comes under grooming & abuse ..
A mature 18 year old body doesn't equal to a mature mind ..
Treating someone with a severe cognitive disability as a romantic partner is an exploitation of their disability.
If any writer forces a character like that into a romantic setting, the narrative will be perceived as portraying or normalizing the exploitation of a very very very vulnerable and a disabled person.