I don't see why they wouldn't allow you the concession, and if they don't, then its sex descimination. Most organisers are reasonable if you talk to them.
Do you have any experience of anybody refusing you your rights as a pensioner concession at any festival.
If so, name and shame here. I am sure things will change quickly then.

However, if the concession is for people over 65, but not mentioning pensioners specifically, then maybe they have a techinical point I suppose.