My co-leader attended a panel at the LLLI conference, which stated that
the immune system doesn't finish maturing until age 6.  I wasn't at that
panel, so I haven't got the reference, but I might be able to get it from
her.  This age wouldn't be all that surprising, of course, since it is
also the age at which children lose their "milk teeth".  From my own
humble experience as a person and as a mother, kids tend to get sick
frequently at this age, but I suspect that is from sharing germs with so
many other kids at a new school...

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