whiz kid
A person, usually a child or young adult, who is exceptionally skilled or intelligent. He was developing software by the time he was 10 years old—he's a total computer whiz kid. Those whiz kids at the spelling bee are always so impressive to watch. Sarah's the new whiz kid in accounting. She'll help you tackle the budget in no time.
a ˈwhizz-kid
(also ˈwhiz-kid) (informal) a person who is very good and successful at something, especially at a young age: ‘Who’s the new manager?’ ‘A financial whizz-kid from Harvard Business School.’