A person can give valid consent for routine clinical examination if he is above the minimum age of

Section 90 in The Indian Penal Code "Consent known to be given under fear or misconception.--A consent is not such a consent as it intended by any section of this Code, if the consent is given by a person under fear of injury, or under a misconception of fact, and if the person doing the act knows, or has reason to believe, that the consent was given in consequence of such fear or misconception; or Consent of insane person.--if the consent is given by a person who, from unsoundness of mind, or intoxication, is unable to understand the nature and consequence of that to which he gives his consent; or Consent of child.--unless the contrary appears from the context, if the consent is given by a person who is under twelve years of age." Ref - Krishan Vij textbook of forensic medicine and toxicology 5e pg - 370 , 421