Motherwell had the first shot at goal, Tom Ritchie smothering Semple's twenty five yarder at the second attempt. But it was Aberdeen who drew first blood in the 6th minute with a corner routine straight from the training ground.
Connor Barron cut the ball back from the flag kick to Tyler Mykyta who floated the ball in for Kyle Dalling to head powerfully in via the underside of the bar.
The Fir Park side enjoyed a decent share of possession without threatening while the young Aberdeen side were at times guilty of trying to make things happen too quickly when perhaps a more patient approach would have paid dividends.
On 16 minutes, Connolly saved well from Barron's free kick but from the resulting corner he was left helpless as Dalling again rose well to powerfully head his and the Dons second from Barron's set piece delivery.
Aberdeen started to turn the screw and Dangana dragged a shot wide before Duncan's excellent cross from the right forced a fine clearance from Johnston inside his own six-yard box.
It was three and easy for the Dons in the 21st minute when Dangana charged down an attempted clearance from Connolly after a short back-pass to net from close in - a real disaster for the Well rearguard.
Another rehearsed corner routine saw Mykyta shoot over then an audacious attempt from Dangana had Connolly stretching to save. Shanks drew another good save from the keeper after a delightful through ball from Max Barry as Aberdeen threatened to take total control of the match. On 35 minutes Dalling nearly completed a hat-trick of headers from corners, but he nodded just over.
Shanks turned well to create a yard of space for a shot from twenty yards but lifted the ball over, while at the other end Starrs had a rare shot at goal as the Steelmen tried to get themselves on the board.
Starrs forced a good save from Ritchie just before the break which came as a relief to the visitors after a very dominant opening 45 from the Dons.
Half time - Aberdeen Under 18s 3 Motherwell Under 18s 0
Brown tested Ritchie in the opening minute of the second half, the keeper palming the ball up in the air before holding it safely. Five minutes later, Lewis Duncan and his Motherwell namesake were both booked on the say-so of the stand-side assistant referee after an innocuous looking coming together.
Full back Johnston joined them in the book after a poor challenge on Kevin Hanratty. Barron was next to fall foul of the ref being cautioned for dissent after a foul had been given against him to the bemusement of most in the ground.
Aberdeen were still looking comfortable but were not offering as much in an attacking sense as the game had become a little bit stagnant.
The booking tally continued to rise as Semple saw yellow before Dangana had a shot deflected away for a corner. From the flag-kick, Dalling's shot was net-bound before it took a nick off a defender to go just wide.
A really nice sweeping move from the Dons ended with Shanks firing wide before a stramash in front of the Dons goal saw loud shouts for a penalty fall on deaf ears after the defence had blocked a series of Well attempts.
Ryan Duncan, Luke Turner and Jack MacIver were introduced for the last ten minutes as the Dons looked to end the game in a positive fashion. Both sides battled hard, but the quality had dropped from the game and it ended with Aberdeen deserved winners after a blistering opening twenty minutes.
Full time - Aberdeen Under 18s 3 Motherwell Under 18s 0