The young Gers were more than a match against a more experienced Bees outfit throughout the 90 minutes, but two goals in the final 11 minutes took the game away from David McCallum’s side.
Indeed, the home side lined up with two 16-year-olds in the starting lineup as Calum Adamson was given the nod in midfield and Josh Gentles retained his place in the forward line.
And the Light Blues started brightly when Adamson strode onto a loose ball at the edge of the penalty area and had a goalbound effort denied by a heroic Brentford block.
The teenager was then at the heart of the action when he dragged an effort wide from 20 yards out that had the Bees goalkeeper scrambling across to his near post.
In the meantime, the visitors cannoned a strike off Kieran Wright’s post when Ethan Brierley opened his body and planted a strike towards goal that looked destined for the bottom corner.
James Graham also went close for the Gers in a first half that contained plenty of positive approach play from both sides without the end product to match.
The hour-mark prompted a triple substitution from McCallum’s side as Kristian Webster, Chris Eadie and Findlay Curtis were introduced.
Indeed, the latter then combined in a neat give-and-go move with Paul Nsio two minutes later down the left-hand side that looked primed to break the deadlock, but the Englishman’s effort was palmed away by the Brentford goalkeeper.
Curtis himself was next to test his luck when he picked up a second ball from a cleared free-kick, although his rising effort never truly troubled the Bees goal.
For all of Rangers’ endeavour, however, they fell behind 11 minutes from time in cruel circumstances after goalkeeper Kieran Wright was caught in possession inside his own six-yard box and substitute Ashley Hay had the simplest of tasks in knocking the ball into an empty net.
The Light Blues ought to have found an immediate riposte when Paul Nsio galloped into the penalty area following a smart pass from Thompson Ishaka, but the 17-year-old lifted his near post finish agonisingly over the crossbar.
Rangers continued to pile forward in search of the equaliser that they merited at the very least, but they were caught out in stoppage time when Brentford were awarded a spot-kick.
Brierley assumed penalty duties for the west London outfit and made no mistake from 12 yards, reversing his penalty into the bottom corner to leave Wright with little chance and seal victory for the Bees.
RANGERS B: Wright, Hutton, Allan, Grant, Fraser (Webster, 59), Strachan (Burnside, 86), Adamson (McClure, 81), Nsio, Stevens (Ishaka, 65), Graham (Curtis, 59), Gentles (Eadie, 59)
Subs not used: Halliwell, Kerr
BRENTFORD B: Valdimarsson, Zabeli, Adedokun, Kim, Arthur Urinboev, Brierley, Morgan, Owen, Yarmoliuk, Young-Coombes
Subs: Angelini, Booth, Dickov, Waruih, Hay, Laidlaw, Lisbie, Headman, Yogane
REFEREE: Thomas Denholm