An intellectual disability is defined by whether a person meets three specific criteria. First, they must have an IQ lower than 70–75. Second, they must exhibit significant limitations in two or more skill areas. Skill areas are what we think of as daily tasks that we develop and use independently, such as personal hygiene, the ability to read or write, social skills needed for group activities like school or work, and personal safety. Finally, to qualify for the diagnosis, the individual’s condition must present itself before the age of 18. Beth shows signs of an intellectual disability almost from birth, but it is not until she is in her pre-teens that her behaviors really start to differ from her sisters’. As an adult, she displays many weak skill areas (particularly around her health and personal safety) that make it difficult for Rachel and her family to support her in living independently.  

Simon talks about mental age as a frustrating and misleading concept. She says that people who don’t know Beth always want to know what her mental age is, as if that would tell them everything they need to know about her. They equate intellectual disabilities with a childlike state and want to know what part of childhood Beth is emotionally and mentally stuck in.