Hey there,
I have two boys 6 & 10, neither have underarm hair. Seems a bit young for sure, but I seen a twelve year old at my kids school last week with a full mustache..lol.
We are watching evolution at a quickened pace it seems. Girls are filling out earlier, so it doesnt surprisde me that boys are also developing earlier as well :).
Zizy,
Let nature take its course and stop worrying. There is no rule of thumb..
Phil