This isnt gospel, just my understanding so hopefully someone will know for sure.
I think the turnstiles light turns green when accepting a "normal" ST card and amber for a concession. The concession doesn't distinguish between old uns and young uns, just tips the stewards to look out for people buying a concession ST when they should be paying full price. So yes, should be ok, unless its a family stand issue where I think you have to be accompanied by a kid