United's demise was completed in the second half when Patrik Berger obtained only his second goal for Villa when he finished off a superb five-man move.
Villa, who had only once previously scored three goals in a game, ended their home programme with their first win at Villa Park in six attempts as they extended their unbeaten run to eight games to the delight of manager Martin O'Neill.
In an almost carnival atmosphere, generated by the appearance of their 1982 European Cup winning side, Villa were more positive than United and scored one of their more conclusive wins of the season.
The signs are very promising for Villa but United are facing an uncertain future with so much at stake in their final game.
Earlier results undoubtedly put added pressure on Sheffield United as a draw would almost certainly have ensured Premiership survival instead of going into the last game against Wigan before their fate is decided.
Typically they played with a great deal of heart and endeavour but always appeared to be chasing a game in which Villa generally held the upper-hand.
Keith Gillespie worked tenaciously but to no avail as United were always playing second fiddle.
Villa swamped United's defence in an attempt to gain the upper-hand and it was not until the 25th minute that Agbonlahor broke the stalemate with a solo goal.
The young striker collected a throw-in from Wilfred Bouma, shrugged off a challenge from Matthew Kilgallon and scored with an angled shot into the top, far corner of the net.
United then survived a hectic spell including one barrage which included three attempts on goal by Agbonlahor, a blocked shot by Gareth Barry and a reckless high shot from Craig Gardner.
But the game was settled in the 42nd minute when Young, with razor sharpness, beat Chris Armstrong on the outside and slammed a low, raking drive through the legs of goalkeeper Paddy Kenny.
It was only the youngster's second goal in 12 appearances since his £9.5million move from Watford in January and it was also one of his best performances.
This goal subdued even the most vocal of managers, Neil Warnock. He must have realised that on this occasion his side was fighting a lost cause.
An indication of Villa's supremacy came in the 54th minute of the game as Chris Morgan produced United's first goal attempt - and that went wide of the post.
Any hopes of United staging a revival ended on the hour when Bouma set up Gardner. He pushed the ball forward to Berger, who played a one-two with John Carew, before slotting his shot past Kenny to complete a performance which could earn him a new contract.