Bearded dragons can be accurately sexed around 6 months of age, though some breeders can do it much younger. As I haven't sexed alot of dragons, I don't know the efficiency of doing this, but deffinitely by 6 months of age, you can be nearly 100% accurate.
How to sex dragons is pretty easy. With the dragon lying on a flat surface or your palm flattened out, facing away from you (teh dragon that is), lift the tail up to about a 90 degree angle and look near the vent, on the tail side, while you gently twist the tail side to side. If you see two mounds on the tail side of the vent, you have a male. If you see no mounds or one big central mound, you have a female. Unfortunately I don't have any pictures...and I can't remember the site that does right now. Hopefully another forum reader will remember and post a link for you.
-----
PHLdyPayne