Ответ:
1. John is my cousin. — Who is my cousin? What is John?
2. He is only 18, but he is already a student. — How old is he?
3. John is very intelligent and he is a good-looking boy too. — Who is very intelligent and a good-looking boy?
4. Many girls admire his dark brown eyes and curly hair. — Who admires his dark brown eyes and curly hair?
5. The only problem is that John hasn't got enough money. — What is the problem?
6. He likes books but he often has no money to buy them. — What does he like?