Unfortunately there is no way to judge age but history... i did have a hen that we didn't know. We suspected she was under a year when we first got her as she didn't molt the first year and miffed for us the first time much later the second year then she we got her the prior. But she was full grown and laying when we got her. That was our best guess, we shall never know.
If your rooster molted last winter he is at least over 18months. If he did not molt last winter he is probably under 18 months old... but unfortunately all my research on the subject that's the best I can do