United's youngsters made a stunning start to the Northern Ireland Milk Cup, hitting New Zealand's Otago for four at the Showgrounds in Limavady.
The Reds, bidding to reach an eighth Premier final, were quickly ahead through sharp finishes from Federico Macheda and Evandro Brandao.
Nicholas Ajose and Oliver Norwood added further strikes after the interval to emphatically lay down a marker for the rest of the tournament.
United are in action again on Tuesday evening, and victory over County Armagh in Coleraine should seal a berth in the quarter-finals.
County Armagh scared holders Fluminese before succumbing to a narrow 2-1 defeat on Monday, and the Reds will need to be on top form again if they are to take a huge stride towards the last eight.