How functional can you be with a low IQ?!

How functional can you be with a low IQ?

I am writing a story and I need to know what someone with severe learning disabilities' IQ would be. I know 120 is the average, but how low is too low. Like what is borderline, a bit smarter than Forrest Gump or something.

"The words, mild, moderate, severe and profound are used to describe the degree of learning disability that a person has. One way to establish the ‘degree’ of learning disability is to use the measure of IQ. This measure suggests that people with an IQ of less than 20 will be described as having a profound disability, those with an IQ of 20 – 50, a severe learning disability, and 50-70, a moderate or mild learning disability.

However, knowing the degree of intellectual impairment a person has tells you very little about who they are and the kind of help and support they might need. The way people’s disabilities impact on their lives will vary, and affect the nature of the support they might need.

People with profound learning disabilities tend to need significant support with most daily tasks, and will often have additional physical or mobility problems. PAMIS, the network for people with profound and multiple disability estimates that such people can require on average, seven hours each day to meet their basic physical needs such as dressing, washing and eating.

There has been some work (e.g. American Association on Mental Retardation) to develop a classification of levels of support, from ‘intermittent’, through ‘limited’, ‘extensive’ and finally ‘pervasive’. This framework attempts to move away from terms of degree of disability, through to a framework of levels of support. It is likely that for people with profound learning disabilities, this level of support may remain at the same level for long periods of time. For people with mild learning disabilities, it may be much more fluid, and change in response to changing circumstances."