In a closely fought game of few opportunities, the youngsters missed two great chances, only to be hit by a sucker-punch goal with eight minutes lef
Jordan Elvferson missed a great opportunity to put his side ahead ten minutes from the break, when he was on the end of Dale Hilson's cutback only to blaze the ball over.
Then with twelve minutes left, a great pass from Kris Irvine saw Ryan Dow get to the bye-line and from his cross Elvferson's effort was cleared off the line.
Four minutes later, the visitors paid the price for that profligacy when a corner wasn't cleared properly and the ball was headed home by Sean Crighton to give Saints their passport into the fourth round.
Dundee United: Grant, Irvine, R Smith, Shawm, lee, Brophy (Thomson 85), Dow, Stokes, K Smith (Colgan 76), Elfverson, Hilson (Howie 72).
Subs not used: Hindley, L Swanson.