My DS4 who was able to read simple readers at 2.5 started writing early. He has pretty much always written his letters the exact same size and could write on a straight line on un-lined paper.

My DS8 who started reading about 3.5-4, started writing about the same time but no where as neatly as DS4 at the same age.

DD18months has had a perfect pencil grip since about 12-13months. I know the boys weren't scribbling that early. I never showed her how to hold a pencil either, I think she learned from watching us.