Okay, I will answer your questions the best I can, but I will be asking a few others...
No real good way to judge age based on the snake's length. Weight is usually a better way to determine the real size and possible growth rate of a chondro. I currently have an animal in my collection that is over a year old and only about 16 inches long, but have two other animals that are only 5 months old and pushing what looks like 20 inches. The two larger and younger ones are from the same clutch, and the other older one is not related to them at all. Some just grow at different rates.
Color change is the kind of the same way. It will be different for different animals, and will vary even within the same clutch with some starting the change sooner than others. But generally speaking, color change seems to go into full swing at around a year of age, but of course this can vary quite a bit between individual animals and even different color morphs(Biaks for example may take several years and go through several different color phases before finishing).
Are you sure your "wamena" came from the same clutch as the others offered for sale? Also, I don't say this to be mean, but are you sure yours is in fact a wamena-type? I have seen many red juvenile Biak-types being marketed as "Wamenas" as of late. I am not sure of the reasoning why someone would do this, but possibly the wamena-type has become more desirable as of late.
Best of luck with your new snake! And plan on posting any pictures soon? 