The data about critical time periods for language is complicated. The accepted opinion among linguists is that children who are not exposed to language in reasonable amounts by puberty will be permanently damaged. Sadly, it is possible to sort this out from the other abuse feral children suffer due to the wide range of horrible abuse different children have suffered. Happily, pretty much every mildly sane human being talks to their children enough to avoid this problem -- you really have to have extreme and prolonged deprivation to hit the critical period. Merely being horribly abusive to your kids for, say, 10
years, won't do it...
However, there are two other important effects being mixed in here. First, very small babies are busy learning what sounds go in the language they're supposed to speak. 0-3 is a critical period for sounding right (and seeing right), which is why you want to watch carefully for treatable hearing and vision problems. A baby who hears no language until age 3 may speak fluently, but will probably always sound funny.
Second, "normal" speech achieved at whatever age isn't what most people want for their kids. Because of the way that the school systems work in Western countries, a child who learns to speak just fine at the age of 10 is probably at a permanent lifetime disadvantage. The whole deal with the importance of using lots of language with little children is not that they can't pick it up later, but that society will deprive them of the chance to pick it up later.
Finally, there's a big difference between being exposed to language and learning to speak. Some kids speak late for other reasons, ranging from diagnosable neurological disorders (some of them vague, rare, and probably of relatively little importance to day-to-day life) through just being stubborn. Whether or not those kids can be helped by more and different language exposure is going to be different for each and every child, and again, some of the issue is going to be more social than biological. A child that speaks late may be heavily penalized by the school system; then you have to decide which bends more easily and effectively, society or the child. Different children, different social situations, different best answers...