I would say wait until it's a year old before switching to sand. It's not so much age that is the factor, but size. The bigger the beardie, the smaller the risk. By the time a beardie is a year old it's almost full grown.
I say "almost" because beardies can continue growing sometimes until they are two or three before they reach their maximum size. My beardie brumated last winter when he was 2, and hit a major growth spurt afterwards, maxing out at 80 grams more than he had ever been before he went into brumation.
As for sexual maturity, I follow the train of thought that a beardie isn't sexually mature until it is at least 18 months old. It can mate long before that, but physically it isn't good for the beardie or the eggs.
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Amanda
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2.0 bearded dragons (Ocho / Domo-kun)

0.1 kingsnake (Rio)
0.0.1 tarantula (Otto)
1.0 ferret (Playstation)
1.0 cat (Wally)
0.1 dog (Tima)