I starting giving my Oustalet's full grown supers around 4 months. I didn't think they would be able to take them down completely, but they literally munched full grown supers with no problem at all!! My chams were about 4" long at that point though.
There is no age (though I wouldn't suggest mealworms often), the general guideline is no larger than the space between the chameleon's eyes (with supers you can get away with length-wise versus width).
With softer bodied worms like hornworms and silkworms you have a bit more leeway, but we are talking about meals/supers. Just for future reference.
Superworms are better than mealworms. You can try them as soon as you feel that the size of the worm won't cause your cham to choke on it. The diameter of the worm should definitely be a lot smaller than you cham's open mouth, and probably not too much longer than their head so they don't have too much trouble eating it. Choking can and does happen so better safe than sorry!