Yes but not for life. Since bearded dragons grow rather quickly, I would recommend a full size home for anything a foot long. Problem with smaller tanks/cages is that the animal does not have very far to thermoregulate, in this case, it may always be too hot. You will need to use a mercury vaporbulb or a very elevated basking area because no UVB bulb reaches down 21 inches.
How big is your dragon now?