Both goals came in the first half, with Arsenal forward Khayon Edwards handing the visitors the lead just after the 20-minute mark, catching United goalkeeper Elyh Harrison in possession before slotting home from close range.
The Reds replied almost immediately, with Dan Gore's surging run and pass providing a golden opportunity for Mateo Mejia, who volleyed into the the back of the net to draw our Under-21s level.
Following the interval, clear-cut chances were at a premium, with Charles Sagoe Jr going closest for the Gunners when he struck the post before Ethan Williams saw a well-struck effort parried for United.