The only way to tell the age of a full-grown bearded dragon is to ask the person in the pet store. You can determine age more easily in the first year of life by the length of the dragon's body. For example, up to one month old, length is 3-4 in.; at 6 months, 11-18 in.; at a year old, 16-22 in. Beyond a year old, ask the breeder or pet shop.