The fastest approach is to anchor on the correct IQ band for imbecility and then check each statement against it.
In the traditional grading of intellectual disability, imbecile equals moderate mental retardation, whose IQ sits around 35 to 49. The option stating an IQ of 50 to 60 actually describes the mild band (the moron category), so it is the false statement and is exactly what an except-type question wants.
The remaining options all fit moderate intellectual disability accurately: the mental age is about that of a 3 to 4 year old child, the person cannot independently manage self-care and needs ongoing supervision, and the disability is either present from birth or acquired in early childhood. Each of these is a true feature, leaving the mismatched IQ value as the one untrue statement.
\[\boxed{\text{IQ is 50 to 60}}\]