>>950101>At what IQ level does it become impossible or very hard for somebody to write properly?I don't think your question really makes sense. If I have it right, IQ values aren't absolute scores, they show where you stand on the test's median average (IQ 100). In the USA you are legally considered intellectually disabled if you are < 70, i.e. 2 Standard Deviations below the median. This just means about 2% of the population (depending on the SD and extrapolated from the test-takers) are legally retarded, and whether you fall into that category or not.
One of the four components of the WAIS IQ test specifically assesses your Verbal comprehension. Anyone who scores low on that category would probably fit the bill you're interested in, including people such as those with Aspergers who would score very high on Pattern recognition tasks instead. Some papers I've found classify a Verbal Comprehension Index of 85 or lower as a statistically significant weakness from the norm.