United's U18s saw their 100% winning start at the Steel Phalt Academy come to and end on Saturday, as Charlton claimed a narrow victory.
The Blades started brightly and put the Addicks on the back foot, but despite a number of inviting crosses from Mallon, United failed to make an early breakthrough, although after good work from Hallam the visiting goalkeeper produced a timely intervention to deny the onrushing Slew.
With their first attack the visitors won a penalty, with Cummings adjudged to have committed a foul, but Charlton's captain fired high and wide from the spot.
Despite that miss, it was Charlton who took a lead into the break after a number of fortunate bounces on the edge of the area broke their way, before the unmarked striker slotted home.
United were again quickly out of the traps and had a great chance to level in the opening minutes of the second-half as Maw found himself in the clear following some good play down the left, but that miss ultimately proved to be the hosts best chance of an equaliser.
Charlton defended deep and invited the hosts on to them for much of the second period and despite the best efforts of Gordon, Slew and substitute Burton amongst others, the visitors saw the game out to inflict a first home defeat at Shirecliffe for United's youngsters this term.
Blades: Ramsdale, Charlesworth, Brown, Gilmour (Ford 83), Cummings, Semple, Maw, Gordon (c), Slew, Hallam (Greaves 72), Mallon (Burton 30).
Unused; Adebowale, Slater.