Don't normally join forums however, I am so fascinated by these little guys and the people who love them this seemed like a good time to change that stand point of mine. I got my first beardie on March 17th and got over my first issue of him not eating (apparently he didn't like the phoenix worms so much). He is such a joy and soooooo tiny. His name is Koopa. Measured him today and he is 4 and 3/4 inches long. NO idea how old he is. If anyone has a clue based on the picture provided or his length that would be great but anyway just thought I would put this up.
Welcome to the world of beardies and beardeddragon.org!! I'm sure you will find many helpful and friendly people here. Beardies are such a joy to own and you will never get tired of them.
As for your little guy/girl I would say if he/she is healthy that he/she is around the 4-5 month old mark.