I was never IQ tested as a child, so I don't know how relevant you'll find this, but in comparison with my brother (who is at best slightly above average) I seemed very, very slow. I could not, and still cannot without the aid of a calculator, do simple addition or subtraction at the age of eight. I could not read or write, tie my shoelaces, dress myself, use a knife and fork, tell the time etc. etc. etc. at the age of eight (although that's when I started learning to do such things).
But I started to rapidly pick up academic skills from then on. I went from being unable to place in year 3 SATs to a mix of 5's and 4's at year 6 SATS, and scoring all 7's in year nine SATs. I have five GCSEs, three A Levels and I'm in the process of completing a degree.
I don't have a diagnosis of anything. I was quite badly neglected between the ages of 6-7 and well, uhm, I suppose it didn't stop until I moved in my partner who helps me with a lot of tasks I can't do, so I think a lot of it was put down to that. Also, one of the areas I was neglected in was being sent to school so I don't think I was there enough for them to notice (Maximum of 60% attendance throughout my school career. I still can't figure out why social services left me with my mum, but oh well).
The point is, intelligence and functioning are mutable. A person who appears to not function well and be of low IQ at five may well catch up fine, particularly given appropriate assistance. Especially when they're not treated as lesser for their ostensibly 'lesser' capabilities.