Redhappy, my DS2 was DXed at 3 with ASD, and had very delayed and disordered speech. Because he went to a SS from 3 - 5 yo, DS1 just grew up around DC with all sorts of SN, Downs, CP as well as ASD, so he just knew and always made allowances.
I didn't sit down and tell DS2 until he was in Y6, just in case he heard things from children at secondary school. Primary school friends had all had an invisible disability talk when they were in infants. (Without DS2) He wasn't fazed at all, and never mentions it unless trying to get out of homework.
I told him that his brain worked differently to most other people, and that was why he was so good at remembering obscure facts/playing computer games (whatever fits, you can big up etc) but found other things harder, like playing with friends etc. That it was called autism, he had autism spectrum disorder. He didn't get that disorder can be a negative term, you might want to call it ASC. I didn't use a book as I couldn't find anything appropriate. He doesn't have Aspergers, he's very different from the AS stereotype and wouldn't recognise himself in any of the Aspie books.
DS3 is 3 years younger than DS2 and I had tried things like, DS2 finds sharing/waiting/talking etc difficult to learn. He never believed me and thought I was being unfair and making excuses for DS2! So I told him the DX at the same time as DS2, and he kind of accepts it.
In retrospect, maybe I should have told them a little earlier, but I really didn't think DS2 would understand any younger. He is quite HF, MS school, middle sets in comprehensive with 1:1 despite his early DX and speech delay, but really obviously autistic, flaps and squeaky high pitched voice etc. Hard to have any sort of conversation with.