Erwin hit both goals in the space of one second-half minute to put the United youngsters into a 3-1 lead at Thorp Arch after Souleymane Doukara had opened the scoring just three minutes into the game.
Doukara, starting for the U21s as one of three permitted overage outfield players, brought the ball down with a deft touch before rifling an unstoppable strike into the top corner.
The visitors managed to equalise midway through the first half, though, as Axel Prohouly fired beyond Ross Turnbull to level the scoring going into half-time.
But, just four minutes after the restart, Erwin put United back in front from close-range, guiding home Doukara’s low cross after a bursting run down the right from the French forward.
And the lively Erwin bagged his second less than a minute later, twisting and turning inside the area before hammering into the roof of the net to give the U21s what appeared to be a comfortable two-goal lead.
Paul Hart’s side pushed for a fourth, with Doukara and Frank Mulhern both coming close to getting it, but QPR pulled one back on the hour mark through Reece Grego-Cox’s penalty after a Turnbull foul.
Then, just four minutes from time, the visitors pulled themselves level through Osman Kakay’s low strike to snatch an unlikely point at Thorp Arch.
Head of Academy and U21s coach, Paul Hart, said: “I’m disappointed with the draw but delighted with the performance.
“We couldn’t have done much more to win. If all our chances had gone in then it would have been a cricket score.
“But you pay the price for not finishing and, ultimately, you shouldn’t have to score three goals to get a victory.
“We were doing an awful lot of good things – we were on the front foot, playing with three strikers.
“The senior players who came in showed wonderful attitudes and set a great example to the kids.
“We’ve upped the tempo in our performances and in our training by a huge percentage. It’s working to a degree and we’ve now got to make sure we get more wins than draws.”
Under-21s team vs QPR: Turnbull, Coyle, Skelton, Vann, Denton, Purver, Phillips, Sloth, Mulhern (Walters 88), Doukara, Erwin.
Subs not used: Peacock-Farrell, Lyman, Parkin, Stokes.