developmental age

It can be both. An autistic child with average or above average intelligence can have sensory difficulties in the classroom that may slow their development, as defined by tests. Autistic children without intellectual disability may have specific learning difficulties, such as dyslexia and dyscalculia, which may also have the same effect when  they are tested for reading age or mathematical proficiency.

This is for school age children, for younger children a delay in speech or a regression in speech can be caused by autism, with the child later catching up and proving to have average or above average intelligence. 

The majority of autistic people have average or above average intelligence, but the proportion of autistic people also having intellectual disability is higher than in the non-autistic population.

My Son is moving to a specialist setting in September  . He shows many ASD traits, he has speech and language delay and is a year or two behind academically. While the SENCO and Head teacher at his previous mainstream school had him down as being moderately to severely intellectually disabled, my Sister who is a SENCO at a different mainstream school has said he is not intellectually disabled he may possibly just not top of the class academically but then many people aren’t.

I think that you would need assessment by specialists to be sure either way. Dr Temple Grandin was late in speaking and her title is enough to show that she was never intellectually disabled.

I think that you would need assessment by specialists to be sure either way. Dr Temple Grandin was late in speaking and her title is enough to show that she was never intellectually disabled.

Temple Grandin is one of my very few "famous" heroes.

I think she is a fabulous human and a true genius.

Not really relevant (so sorry for butting in) but I can't see her name without letting off a metaphorical flare!