Hi there.....size will be a factor, and you can generally tell the age within a few months with beardies that aren't full grown [ with some exceptions because not all dragons grow at the same rate or to a large size] so it will always be a guess even with a young dragon. For an adult like yours it's very hard to tell, but a younger adult will generally be faster + more active unless he's not well for some reason. So it's not really possible to tell for sure.