I'd go with a couple possible options --
- he hits at a parental insecurity for that parent b/c he is academically more advanced than the parent's child and the parent has bought into the idea that parenting is what makes a child achieve highly or be gifted;
- the parent doesn't know that he is younger and the comment was more directed toward him being underplaced with the assumption that he is older and you are holding him back with younger kids in order to fulfil your own parental ego or something else. (Of course, if they mentioned his age, this is unlikely);
- the parent is unaware that he is advanced and doesn't know why a younger child is in the class.
