It's the age of the child, not the height that counts. Anytime your spouse (married, divorced, or separated) travels internationally with a child but not the other spouse a letter should be in hand from the non-traveling parent. Now, they generally will be more lenient with older children who can speak for themselves but just because a child wants to go with the parent doesn't mean that parent has legal custody.

On a related note, friends are traveling from Canada/USA (cant' remember which direction this occured) with their children and a friend's child. They had a letter from the other child's parents. BUT, this child "jokingly" makes a comment about being kidnapped. Took several hours to sort that one out.