If you go onto YouTube and search for Amanda Baggs and watch her video called 'In My Language', that might give you something to think about. Basically she is non-verbal, however she does use a computer to communicate.
My impression when I first watched the video was that she had learning difficulties and if you met her you would probably have very low expectations of her capabilities. When she communicates through her computer she literally blows away your previous assumptions as to her ability. But I'll let you watch it yourself and then post back with what you think.
I think alot of parents (and professionals) think non-verbal = less intelligent. That is also because alot of the testing of 'intelligence' and 'capability' relys on understanding and using verbal communication. Yet it has been demonstrating time and time again that if children with autism are given non-verbal tests they tend to come out with remarkably high results.
My son for example is verbal. But he has difficulties learning because alot of the autistic traits seem to affect him specifically in this area. I know another mother whose son can read, write and do some basic maths, yet he is non-verbal (he is 7).
I don't think there is any age that you will be given a 'non-verbal' diagnosis. I think the diagnosis will probably be autism and it will be noted that at this stage the child is non-verbal. Being non-verbal does not mean that they don't understand what you are saying. They may well be able to understand you and be able to read etc, but for some reason they either can't (or with selective mutism, won't) communicate verbally.
If a child is non-verbal then they tend to use picture exchange communication systems and sign language.
Will there be more problems with behaviours? I don't know the answer to that. If you cannot communicate then you are more likely to get frustrated or angry. So I think it is very important to use the other ways of communicating straight away. Using signing or PECS will not delay the start of speech. If the child can communicate and wants to communicate then they will.
Is it considered more severe? Probably yes, but that is from our perspective and being able to use speech to communicate and interact is classed as a pretty essential function. Is it an indication of intelligence level? No.
I have heard of children up to 9 being non-verbal before they started to speak. There may be other parents who can post with more information on that.
Do they make any sounds or words at all. Yes they can.
If the child was echolalic, but using learnt phrases or advert jingles etc to communicate or interact then the intention to interact is there. I'm not sure where the dividing line would be. You may need to get clarification of that from an Educational Psychologist.
If you google echolalia you should get some good definitions of this.
Echolalia is basically repeating things already heard. But that can be used in a non-communicative way or a communicative way.
I believe a lot of nonverbal autistics talk, but it doesn't leave the head unless one can be taught to communicate by other means than speech. It's just a guess of mine, I speak a lot more in my head than I actually vocalize. I believe I speak a lot better in my head than verbally. Verbally I perceive myself as average though I speak an occasional to frequent stammer (especially if I try to speak as I think)...people seem to think I talk really well and ask me questions about what college I went to, etc... I guess they don't notice my stammering as much as I do.
I'm sure there's a range in the internal verbal abilities of people, but overall I believe there's a lot more going on in people's minds than what appears on the surface.
I'm not surprised when a supposed non-verbal autistic person, previously thought to have next to no thinking ability, pops out a well composed essay or document when given the training and the chance. Someone nonverbal I'm sure gets plenty of time to reflect and think upon lots of things. (As well as time to be very frustrated about not being able to share and express those thoughts).
Here are some interesting articles to read:
I thought I linked to these in another post but I can't find it.