Words have meanings so it is up to us to use them correctly. Which do you think applies?

Ignorant
lacking knowledge, information, or awareness about something in particular.
"they were ignorant of astronomy"
synonyms: without knowledge of, unaware of, unconscious of, oblivious to, incognizant of, unfamiliar with, unacquainted with, uninformed about, ill-informed about, unenlightened about, unconversant with, inexperienced in/with, naive about, green about; More

Retarded
less advanced in mental, physical, or social development than is usual for one's age.