I think chinchillas are an adult by year 2 or 3? I think my chinchilla, Baby X, is about three or four now, born a week before or after Christmas 2009 or 2010 and I got in him Feburary. I have such a bad time with keeping up with time xDDD! But he still has a really baby face! Big eyes, a face/head like he's still about 1 years old, and childish attitude.
My mom's, Shadow, is about four or five. We had no idea when he was born, but he looks so much older and wiser than X. His eyes are smaller and he has a more mature looking face/head. But he and X both look like they're around the same size, especially when they're spread out. Maybe 12 inches long average or so?