Is a minor, undiagnosed condition a pre-existing condition?

You are in the gray area. Insurance companies consider something pre-existing if you've seeked medical treatment or advise or if "a prudent person" would seek medical treatment or advise. This gives them an out. They will ask about it in the health questions. If you answer truthfully they may decline based upon an undiagnosed condition. If you don't mention it and they find out they can rescind the policy. Chances are you could slide through with no problems but there is the possibility of getting caught.