There's no hard and fast rule. Many six year olds need to learn the difference between singing and speaking. Their range is very limited- usually E or F to B or C. 11 year olds are near the peak of their children's voice, and can sing from about a B-flat to E or F if they are experienced with singing. 13 year old girls can stretch a bit more, but the boys often start to change at that point and have a lower, much more restricted range. The unchanged boy at that age can have the purest, sweetest voice around, as long as he hasn't spent the previous 10 years at sports events, screaming his head off. Same for girls!
Read any books on the Kodaly method- they lay out a specific scope and sequence for teaching children to sing and read music. "The Kodaly Context" and "The Kodaly Method" are both great.
< href="http://oake.org/"> OAKE (the US Kodaly society) and
The Kodaly Society of Canada
are two good links. Click on the blue letters and you will go directly go their pages (lord willing).