In spite of the assumption that late speech equals autism, a small percentage of of non-autistic children actually don't speak till age 3 , even as late as age 5 (language delay and speech impediment, including verbal dyspraxia is common and is not autism). A number of people with Asperger's also speak late, in some cases because of selective mutism, in others because of verbal dyspraxia (articulation/pronunciation problems). Unlike most people with autism, however, they go on to develop normal speech.
By contrast, around two thirds of people diagnosed with autism don't develop verbal language and around one third develop language which remains highly idiosyncratic or 'dysfunctional'. I was in this second group, able to speak but not having fluent functional language until late childhood. Whilst I could read fluently, it was found I could understand some words with meaning but not sentences.
I developed the ability to understand spoken and written sentences after the age of 9. Whilst I could chatter to myself I then began to try and speak in sentences in front of people using their 'interpretive language' from the age of about 11 and by 13 had some reasonable level of communication. Many who remain functionally non-verbal have been able to communicate through typing. The articles on this communication section may help explain some of the broad diversity of communication challenges affecting different people with autism. I have also written extensively about this diversity in my books.