There are physical changes that can cause a Girl to be called a woman and the same is true for boys becoming men.
There there are laws that says when you are legally a man or woman.
However, I have known members of both sexes who continue to act like children, even after meeting the legal age and going through the physical changes. Some people just never grow up.
As far as the sportscaster who called a 19-year-old a girl instead of a women, he made a mistake, unless the person was competing in a match for girls that cover an age group of 16-20 for example.