Arsenal booked a League Cup fourth-round tie against London rivals Chelsea by holding their nerve in a penalty shoot-out at West Brom.
Misses from Craig Dawson and Morgan Amalfitano swung the tie Arsenal's way with full-back Nacho Monreal converting the decisive kick.
The Gunners' Thomas Eisfeld had applied a cool finish to Nicklas Bendtner's pass to score after a dour first half.
But the lead was short-lived with Saido Berahino nodding in Steven Reid's ball.
West Brom had came close to a decisive goal in extra-time but Amalfitano hit the bar and saw an effort cleared off the line as he came within inches of saving himself from the spot-kick pain to come.
Instead it will be this new generation of Arsenal youngsters who take on Chelsea on 28 October and bring Gunners boss Arsene Wenger back into opposition with Blues manager Jose Mourinho for the first time since the Portuguese's return to England from Real Madrid.
Wenger and Baggies counterpart Steve Clarke both named much-changed sides from their weekend Premier League games, with Arsenal giving Mikel Arteta and Bendtner starts alongside less experienced team-mates, and a disjointed first half followed.
Reid's cleanly struck free-kick demanded a stretching stop from Arsenal goalkeeper Lukasz Fabianski in a rare moment of excitement, but the second 45 minutes proved far livelier.
West Brom appeared to be gaining the upper hand with Shane Long and Berahino threatening, before Arsenal halted their momentum with a slick opener.
Bendtner's precise pass pulled apart the hosts' defence and full debutant Eisfeld showed steady nerves to send goalkeeper Luke Daniels the wrong way and find the net.
West Brom restored some justice to the scoreline within 10 minutes.
Berahino, who scored a hat-trick in the previous round against Newport County, found a pocket of space in the much taller Arsenal defence to head home.
The England Under-21 striker fired over the top before the game headed to extra-time and, despite an entertaining half hour in which Arsenal debutant Hector Bellerin almost scored a memorable winner and Amalfitano went close, the sides could not be separated and had to decide the game from 12 yards.
Serge Gnabry's tentative effort on Arsenal's second penalty was smothered by Daniels but, after confident conversions from Reid and Morrison, Dawson and Amalfitano's misses gave Monreal the chance to stroke home the decisive kick and spark gleeful celebrations.
Arsenal manager Arsene Wenger:
"I am proud. We played with many young players and they showed they are not only good footballers but that they can fight as well.
"I decided to go for young players with the penalties and they coped well with the pressure.
"Of course, penalties are a bit of a lottery, but overall I think the most important thing is that we played well in the game and that the young players came out and showed that they can play the way we want to."