One note of caution should be sounded at this point. Child language researchers
certainly report very carefully on the age of any child whose language they study.
However, they are also very careful to point out that there is substantial variation
among children in terms of the age at which particular features of linguistic development
occur. So, we should always treat statements concerning development stages
such as “by six months” or “by the age of two” as approximate and subject to variation
in individual children