Bearded dragons live 8 to 12 years when kept as pets. Telling a beardie's age based on size is easy to do in the first 2 years of life, but there's no reliable way to tell the age of a fully mature dragon other than finding out the dragon's date of birth from your breeder.
The First Six Months
You can estimate your juvenile bearded dragon's age by length from the tip of his tail to the tip of his snout. Hatchlings begin life at less than 4 inches long. By the time bearded dragons are 6 weeks old, they'll be just over 6 inches long. The dragons grow at just under an inch per week. A dragon's whose head to tail length is creeping up on 17 inches is approximately 6 months old. Females' growth rate slows in the last month; they may be as much as an inch shorter than same-age males at 6 months.
You'll know your beardie is at least 6 months old when sexual differences become obvious. Males develop enlarged pores for scent marking on the insides of their front legs. When you lift a beardie's tail, look at the base of the tail above the vent opening. A female or a sexually immature juvenile has a narrower tail, with a single bump above the vent opening in the center of the tail. Sexually mature males develop two bumps, know as hemipenes, above and to each side of the vent opening. By the time bearded dragons are fully mature at about a year of age, they'll have reached their full size of 22 to 24 inches.
Factors Affecting Growth
Pogona vitticeps is the most common -- and largest -- bearded dragon kept as a pet. Other species can be much smaller than the 2-foot-long vitticeps specimens. P. minor mitchelli and p. minor minor are two medium-size species that top out at a maximum size of 18 inches. Small dragons that stay about a foot long include p. minor minima and p. henrylawsoni. The tiniest members of the family, p. microlepidota, may reach just 4 to 6 inches in length.
Environment and Care
Whether your bearded dragon attains full size depends upon how you take care of him. If his environment is too small, he'll stop short of full growth. Undernourished juveniles stay smaller than their well-fed counterparts; those who are ill due to poor cage hygiene or lack of full-spectrum lighting look puny compared with healthy bearded dragons.