When very young children draw they generally think it's an accurate image, they don't process any kinds of comparison to reality, adults are more easily able to notice when something is incorrect. So while an inexperienced adult might draw a totally disproportionate image, you're not going to find one that draws arms coming out of hips or anything of that degree when attempting realism. It's more to do with brain function in young children, certain parts don't mature until later stages of development.
Matter of practice. An adult can still achieve respectable level of art skill, even though a child will get there sooner. As for drawing styles hmm. I don't remember 7 year olds drawings of detailed hardcore lesbian sex scenes so I'd say yes, they do differ.
I admit, the hardcore lesbian sex scene example may be bit off the mark but I'll need to clarify this: Do you mead a style as an individual type of hand movements resulting in unique images OR the level of proficiency?.
I keep having this argument with adults who draw like children.