I've seen a great number of children that just talked later. But some of the time there were other reasons. In some cases the children had a lot of undiagnosed ear infections that interfered with their hearing. Other times it was that they actually had hearing problems. But the biggest problem I see is that parents don't realize they are baby talking to their children. There are many ways that parents do this. Sometimes it's the constant raising of their pitch when they talk to them. Some parents also shorten their own sentences and use bad grammar when they talk to their kids. A lot of parents will literally mirror their children's own 2 sentences and grunts back to them.
In my daycare we use a lot of language. We take the time to correct the children's grammar. We also will take the 2 and 3 word incorrect sentences they say and as we are getting them what they are asking for we will take the time to say that what you should have said is this... I NEVER talk to them like they are babies even when they are babies. It works very well. Most of our kids talk early and talk well.
I'm reminded now how important a check up at the doctor is though. I have a little girl on nights I just started that is 3 and can not communicate at all. She couldn't see or hear most of her little life so far. She is like taking care of a very young toddler. She has hearing aides now but will have to start over from scratch. I don't know why this situation was allowed to go on for so long.
Suzi