Just out of curiosity, I attempted to buy a single from Heathrow to Hayes and Harlington with a 16-25 Railcard from a Heathrow Express office the other day, and I found out that not only does their particular system disregard the Railcard for pre-0930 journeys but it also assigns a £8 minimum fare thereafter... 
(And no, the LUL ticket offices there don't sell such tickets either, with or without conncessions)
(And no, the LUL ticket offices there don't sell such tickets either, with or without conncessions)