Worth keeping in mind that Helen Keller didn't loose her hearing (and sight) until an illness at 19 months old.

At this age, a child's brain has already locked in the sounds for their native language and lost the ability to learn non-native sounds (hell, research suggests that unborn infants can recognise the difference between their mothers native language and foreign languages before they even leave the womb). The typical child will have been using single word sentences for months and just starting to move onto two word sentences.

Keller might have regressed to zero language abilities after her illness, but she didn't need to start completely from scratch when she learned how to speak.