Of which you have recieved several already. My first inclination was to say most 6 year olds range between 2191 days and 2556 days old - but realized you did mean vocal range.

6 year olds are mainly going to have a treble range - the 12 to 15 year olds are going to be a mixed bag and so are the 15-18 year olds. Some of your oldest group may have voices that are still settling down to an adult range - others may have more or less stable voices.

the 12-15 year old age group will probably have the most variation in vocal range - and it may vary from minute to minute!!!!