I just started free writes in my beginning classes, after months of stories. The kids love to write them. They love to count their words. One student, over the past three free writes (over a 2 week period) improved from 44 to 55 to 66 words. That will make a nice bar graph to show the parents in the spring, along with the writing itself!
I believe that kids should start writing when they want to. But we do need to have artifacts for parent conferences in the spring. Starting now on free writes with your beginning classes is therefore not such a bad idea. What is required for success in free writes? Stories*. Lots of them. Vocabulary from stories sticks better with kids than vocabulary from novels. Go figure.
*Wall word charts are also very good to spur their writing.