You lot have made me almost wee myself. Boo - I love your description of "a face like a melted welly".
I was the one (hang head in shame) that made a slightly derogatory comment about a child yesterday with regard to their looks, and you're right - I did feel guilty, even though the comment wasn't that bad or that personal. Since then the child has been likened to Joe Bugner and Antony Worral-Thompson... far worse (but debatably true). But she is cute, even if not beautiful and what does it matter anyway?
I suppose an acceptable age to start saying out loud (though not to their face) about a person's lack of beauty, is probably adulthood. Like you said, before that - you get lynched or branded an evil person.