In the Lord of the Rings movies and books, Saruman the white was able to breed an orc army within 6 months. He doesnt start until Gandalf visits him, and is finished went the Ents go to town on his tower. So by this source its like they come into being fully grown?
When I visited the fandom fact pages, there was nothing about them having to grow.
Even in the core rules there is nothing about them having to grow up from children.