Honestly, I always judge by the size of the cham, not necessarily their age as to when to move to a larger cage. There can just be such a difference in sizes that I think that's a better way of judging, instead of a random age. For example, I have a female ambiliobe who is smaller than she should be for her age, so she's still in her baby cage - it's plenty large enough for her now, and when she grows bigger she'll be movin' on up!