I wish I could remember more, but I did once read a touching story in a Reader's Digest (probably in the dentist's waiting room) about a child born with serious medical problems who could whistle before she could talk, I think at about a year old.
A friend of mine performs the Mexican Whistler on flute and I claim that he is cheating.
I've never seen an explanation of how to do that two note trill that uses the angle of your tongue to change. I can do it but I can't explain it myself!