i just got a vailed chameleon 3 months and i got a like two foot high one foot wide cage and it likes it. i would say to change to a bigger cage at about 9-10 months old
Gonna want to go 16"x16"x20", 16"x16"x30-36", & finally 24"x24"x48". Hard to say at what age, as they all grow differently. But around birth - 4 months, 4-10 months, & 10+ months. Trick is for them to still be able to find their food easily. You'll know when it's time.
I got my cham at the beginning of the academic year (august) and he was about 5 months old. I got him a reptibreeze 18x18x36 (1.5x1.5x3 ft) to start off with. I just ordered a new cage from Dragon Strand (one of the forum sponsors) to start getting his adult cage ready. The adult cage is about 2x2x4 ft.
I will use his old cage for a cage I can put him in outside so he can spend some time outside on nice days once it gets nice out.