Appox. ages of BD's?

[Cyrene]

I have two BD's, both were puchased from pet stores. I've had them both for about six-seven weeks now. I'm wondering if anyone could help me figure out how old they are. Also, one is growing very well and the other hardly at all. Fire is about 5 inches SVL and Smoke is 2 3/4 inches SVL. They were about the same size when I purchased them but Smoke doesn't seem to be growing much, even though s/he (?) eats like a pig! They get dark greens and other veggies in the morning and then about an hour later I give them as many crix as they will eat. Fire will eat around 12-16 crix (1/4 inch) and smoke will eat about 5-6. I offer them more crix in the evening, about 3 hours before lights out and they will usually eat 3-4 then. Crix are fed juvenile BD food, potatoes and carrots and dusted with ReptoCal. They are in seperate cages as well, so no competition over food. Is it normal for the males and females to grow at different rates? I don't know what sexes they are since they are so small, but I'm guessing that Fire is a female at the moment. The temps in the cages are good, gradient goes from 75F to 97F with the basking spots at around 110F. Should I be worried that Smoke doesn't seem to be growing much? One more question. They both have just paper towels on the bottom of the cages. When is a safe age/size to add sand subtrate?
Thanks in advance for any info!
Cyrene

[ClementeDragons]

All bearded dragons grow at different rates; I have a 3-month-old bearded dragon that is 5 inches svl and a one month old that is 4.5 inches svl. Yours were most likely about one moth when you bought them because they are very small so I would say about 10 weeks old. I like to start my dragons on sand when they are about 1 month or 8 inches long. Make sure the sand is very fine like screened play sand.