Some years ago, as a graduate student, I had a part-time job to support myself at the Sunland Training Ctr in Gainesville, FL. My job was to do psychological evaluations on the residents -- they were what was then known as the "mentally retarded."
I tested many residents who were, say, 17 to 25 years old but who were no larger than a 10 month old baby. Males had the beginnings of whiskers and looked like a strangely morphed version between a baby and an adult.
They had a very shortened life expectancy -- typically no longer than 25? years at the very longest and typically much less.
Their IQs (100 is the average) were always in the single digits (which would be roughly equivalent to a mental age of less than one year).