Because they are a minor and not reached the age of consent. This is much different than somebody who is otherwise possibly impaired because of drugs or alcohol.

And as far as "sound body" goes are you saying a person who is of poor health or of frail body can not give consent? If this is the case then I know of many handicapped and sick people that would disagree with you. Every person on crutches or in a wheelchair for instance can be of sound mind but not of sound body for just starters. Many people that make last wills and testaments are of sound mind but not sound body that is the whole point of them making the will in the first place because they are about to kick the bucket and they want to get their last wishes down before they go.