It's almost impossible to know the age of a bearded dragon unless you know the exact hatch day. This growth chart may help determine the approximate age but all dragons differ in size: http://www.beautifuldragons.503xtreme.com/Growth.html
It depends on how well they were taken care of as babies and their genetics, among other things. My two are both around 16 inches. Goldie is 16 inches and Rosie is 16 and 1/8 inches. They are both over 2 years old.
I agree that it depends in large part to their genetics and the quality of care. My oldest girl is about 2 but she is only 15 inches. I just got her as a rehome from this site. My 8 month old girls are 17 inches and 16 inches, and will get bigger. My 4 month old is about 13-14 inches already and will be huge.
well I'm taking great care of them, but I can usually only get them fruits/veggies. I really hope they grow more. I just got them in the last 4 months seperate from KSL.com, and the previous owners didn't take good care of either. Rambo was as light as a feather and 14 inches, and Lizzy was in the thinnest sand I'v ever seen and it was blue and it died her blue too. It got all dusty and floated away when it got shook. she's still blue!
You really need to get them feeder insects if you're worried about their weight and growth. Fruits are a rare treat because of the sugar, and veggies just don't have enough protein for bearded dragons to grow.
Mine is only about 5-6 inches right now. Based on the sizes of most of the comments on here, I'm assuming that mine is pretty young - would a couple of months old be reasonable to say?