It seems like after a certain point that is going to invite more scrutiny and questions rather than discourage it because the next logical question is "how old are you, you look young for x grade..."

Our approach has always been to assess what they are asking. Most often when people are asking about grades that is really a stand in for asking age so we just answer that question. Or, at times our son simply says "I'm homeschooled and we don't really do grades."