This is not always the case, although home practice with mom is great.
It depends more on the program than the age of the chid. Some children will not learn anything in a dance class ever. :) Others can be very skilled at age 3. Depends on how the program is structured and how the child responds to training.
However, it is NEVER too late to learn how to dance. I know people who did not learn to dance until 16 and went on to have a professional career.