I'm an adult clutterer, and so maybe I can give some helpful information on
this.

I think that cluttering really is something that you can "outgrow," and
that's been one of the most helpful things for me in helping me to
overcome my cluttering, is to try to learn what I missed earlier in life.

One of the premier cluttering researchers in the field said that he
thought that all children clutter at 2-3, and then they outgrow it.

Also, there are various levels of cluttering. If you meet someone with a
very severe case, they are unintelligible, but that doesn't mean that all
clutterers are unintelligible. I think that cluttering is just as common,
if not more common than stuttering, but since it is hard to identify, and
clutterers generally don't seek treatment, then it seems to SLP's that it
is rare.

I made a website for discussions about cluttering. It is:
http://groups.yahoo.com/group/cluttering/

Everyone is invited to join. Thanks.