small for age...

[damzookeeper]

I have a beardie I bought about 2-3 months ago. He was supposed to be 2 months at the time, but I am pretty sure he was a hatchling. He was about 5 inches at the time and I didn't have a scale then so I don't know how much he weighs.
Anyway, he is housed alone and always has been in a 20 gal. tank. He was thought to be female but was too young to be 100 percent sure. Anyway, I've had him for 2-3 months now and I recently measured him/her and he was only 7 inches and I recently got a scale (Thank you Teresa!!) and he only weighs 12-13 grams. What is wrong with him! fecals came out clear. He eats but not alot. 10 crickets a day, sometimes less, and he has started eating a few small silkies.
Is is something I'm doing, all my other babies are big and nice, could it be genetics or something?