United's Under 21s will miss out on the chance to defend their Professional Development League National title after a tense and fraught semi-final defeat to Brentford in extra-time.
In a closely fought encounter which saw very little separate the two sides on an afternoon where both teams could have scored more, it was Iwan Morgan who in extra-time swept home the winner after Alfie Atherton had equalised for the Blades with 20 minutes of normal time to go.
The visitors started the more dangerous of the two sides, but Coby Hewitson wasn't overly tested as a couple of efforts went off target and another well blocked by Zain Tahir, but United grew into things and enjoyed the better of the final 15 minutes of the first half.
Jevan Beattie saw a low effort well saved and 10 minutes later went even closer when he cut inside his man and saw a powerful effort pushed out by Julian Eyestone.
Billy Blacker and Atherton were then both denied in a flurry of late chances, but that was all undone in stoppage time when a free-kick into the box was headed home by Benjamin Frederick.
United though went back on the offensive straight after the break, Jefferson Cáceres heading an effort straight at Eyestone as the Blades peppered the Bees box with crosses.
Blacker curled over and saw another powerful effort pushed over before Eyestone made his best save yet after Beattie sent Atherton clean through, the goalkeeper tipping his low effort just past the far post.
Atherton did though equalise with 20 to go when Blacker found Waldron with a superb pass, the ball eventually finding its way to the forward who hammered home the equaliser.
As the game became stretched through 10 minutes of stoppage time, both sides went close to winners, but the match moved to extra-time and in the baking heat, gaps opened up everywhere as Aston forced an early save and Waldron saw a header scrambled away.
Waldron then made a huge block down the other end, but minutes later Morgan swept home the decisive goal with just under 20 minutes to go.
United pushed late on but a succession of balls into the box were well dealt with, the only major chance falling to Sam Colechin who smashed an effort over the crossbar with just a few minutes left on the clock.
Blades: Hewitson, Waldron, Tahir, Blacker (Giggs 90), Baptiste (Trialist 85), Sasnauskas, Atherton, Makhalira (Colechin 78), Beattie, Aston, Cáceres (Francis 104).
Unused: Grainger.