Depends on the breeder.
I my opinion they shouldn't be sold before they are 2 to 3 months old ... but I have seen people selling them at 3 or 4 weeks old.
If the breeder doesn't know the hatch date, I would be suspicious of that breeder.
Also it's almost impossible to tell a young chameleons age by it's size.
The size at any given stage is dependent on the environment/food to that point.
I have seen 3 month olds that look 4 weeks old and vice versa.
-Brad