My twin daughters were very delayed in speech, mainly due to the fact they had three older siblings who always knew what they wanted, etc.
When I attended their 2-year old check up, the 'Well Baby' Doctor was most concerned that I replied 'None' when asked how many words they used. Started to tell me about story reading to them etc. The HV - who knew the family well, rolled her eyes at his comments!!!
They started to talk three months later and, within a fortnight, had gone from single words to nearly full sentences. Both ended up with excellent degrees and Master Degrees. So, late talking has nothing whatsoever to do with intelligence.
One of G.sons had what is known as 'backtalk'. His language could not be understood. He, obviously knew what he was saying and became increasingly frustrated with our total inability to understand him. He seemed to have an extensive vocabulary, none of which could be understood. His Mum became reasonably adept at interpretation, but for the rest of us, including his friends at nursery, he was speaking a foreign language.
With a lot of effort they managed to get him referred to S<. Everyone was concerned that he would really suffer when starting full-time school, and his frustration at us all was resulting in some mild violent behavior.
S< gave him two lots of six sessions, diagnosed the problem and worked with him and also gave his parents exercises to do with him to get his speech back on track. Worked brilliantly and a year later, it was impossible to remember the problems. They said it would probably have started to sort itself out around the age of 6 or 7 years old, but everyone, particularly the child himself, is so glad they did not leave it that long.
He is now 9 years old and speaks beautifully. I actually remarked that he has no regional dialect, and his Mum (my daughter) explained to me that is due to the fact that he still has the original problem, but it is sorted out now as when he comes across any new word, he needs to see it written down then he can say it properly. Due to this, he speaks as he reads words, thus really good English!!!!