system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes a brain for someone with a certain condition, to get “stuck” at a mental age much lower than their actual age?
gpt
When someone has a disorder that causes them to have diminished mental capacity, what's going on isn't really that they are 'stuck' at that age.  Someone who might be described as "having the mental capacity of an 8 year old" does not actually have the mind of an 8 year old.

This is simply a convenient, simplified way of expressing how much mental capacity someone has.  It's easy for everyone to understand, because most people have at least an approximate idea of how much mental capacity an 8 year old has.

Disorders that cause diminished mental capacity usually do so because the development of the brain is impaired.  But development of the brain being impaired doesn't mean that the brain is "stuck" at a certain developmental stage.  Consider a physical developmental abnormality like spina bifida.  A person with spina bifida definitely has reduced physical capacity compared to an unimpaired person of the same age.  Their body is clearly not 'stuck' at a younger developmental age, however, it simply has developed abnormally in a way that causes them to be impaired.  Developmental disorders that affect mental capacity are no different.  The "mind of an X-year-old" thing is just a way of trying to express how severe the impairment is.