How does DWP decide whether a young adult qualifies for a Personal Independence Payment? (PIP)

My daughter is 18 and has no mobility issues but cognitively she functions like a much younger child. DWP have turned down her application for PIP because there is 'no evidence of cognitive impairment', despite the fact that she has attended a special school for children with communication difficulties since she was 7, and will continue into specialist further education.

DWP interviewed her and she talked very proudly about her achievements, such as travelling to school independently and flying to Portugal to visit a friend on her own. Of course she doesn't want to/ didn't think to emphasise that she flew in the care of the airlines' special assistance team, whilst her ability to navigate TFL to get to school is the result of rigorous travel training through the local authority. DWP are using this exchange as evidence that she doesn't need help.

She clearly has a learning disability, however the only formal diagnosis she has ever had is for ADHD more than 10 years ago. Her medical notes throughout childhood refer to global developmental delay but I don't think that counts as a diagnosis. Is she entitled to these benefits? Or is this really for physically disabled people? Most of her school-friends qualify, should I get her assessed in order to prove that she needs support?