The earliest age your child can take voice lessons:

Ideally the older the better.  We do take younger children (they have to be at least 8 years old) for private voice lessons, but they need to be aware that they will have to do some vocal technique such as scales and vocal exercises.  If it is not working, in the best interest of the child we will suggest that they hold off for a year or two and then try again.

The changes that happen as children mature physically, such as development of lung capacity, vocal chord changes, etc. will effect how they progress.  Many concepts can be understood young, but not all can be practiced because the body won't cooperate.