I have three dragons and they are each very different.
Gryphon, my oldest, is a beefcake. I got him from Georgia Bearded Dragons at 2 1/2 months old. He is almost 4 years old, 23" and 810 grams. He reached his full length at 18 months.
Boggart, my second oldest, is average size. He was malnourished when I got him at 4 months and after his second birthday, at 15", our vet told us he would more than likely stay small, although he is healthy. Soon after he grew and is now 19" and 620 grams. He is currently 3 years old.
My youngest I have had since the day after he was hatched. He has been fed top quality food and vitamins his whole life, lives in a 40 gal breeder viv, is 9 months old and is only 14" long and weighs in just under 400 grams.