From United's site:
ST/LMTB Holders and Members can gain their usual discount upto 5pm on Tuesday 18th October only by quoting their membership number(s) and a half price facility is available to under 16s and over 65s.Alternatively there will be cash admittance on the evening of the game from turnstile ranges E31 and W15 (Manchester United Supporters only) priced £31.00 adults and £15.50 under 16s / over 65s.