Well first of all you'll need three seperate dusting powders. Calcium with d3, calcium without d3 and a multivitamin like rep cal herptivite. You should dust with d3 only 2x a month, I do every first and third monday of the month. I dust every cricket I feed my cham on those days with the powder, you do not need much just a couple taps on the container and that should be enough. Also if you do not know you use calcium without d3 at every feeding and use the herptivite 2x a month as well.
Also it's very hard to age chameleon precisely, we can only guesstimate how old your cham may be..