The vast majority have some attribute that's not so attractive, particularly as we get older. Too tall, too short, too fat, too thin, grey, haggered, whatever...
It's pretty normal for a child to just say something in all innocence, but related to that attribute. Yes, it can hurt, probably more than many realise, but there's nothing you can do about that is said, only about how you react and deal with it.
I am pretty sure many small children will ask 'why does that person only have one arm?', or 'why is that person in a wheelchair?', and if that is a really sensitive topic, it's going to upset you. But you are not your nose, you are not your wheelchair, you are not your complexion...
You're always a hell of a lot more than a physical attribute. Always. If you can, try to embrace that, rather than fear criticism of a relatively minor attribute.