I belong to two professional bodies, the British Psychological Society (BPS) and the British Society of Allergy and Clinical Immunology (BSACI). The BPS are giving me student membership because I am doing a PhD but the BSACI have told me I can only have a student membership if I am under 35 years of age. Well, I am not! Can I take them up on this, do you think? I am earning £0 and I am a full-time student, have any of you 'oldies' ever heard of this?

Universities do not place upper age restrictions on their students (there are very few exceptions, but the only one I've heard of is graduate medicine where some courses like you to be under a certain age so you can serve the NHS for long enough!). Therefore there is no valid reason why a society should do this when assigning student memberships.

You can demonstrate your student status formally, and it is unfair of the society to discrimate against age. I don't know for sure, but I would be very VERY surprised if, in this day and age, you wouldn't have a case.

In another post recently someone was asking about getting a mortgage when a student. I said I had had no problem as PhD funded study was seen on a par with employment for most lenders. However I have just remembered because of this post re age that quite a few of the graduate mortgages available stipulated you had to be under a certain age - again assuming students are young(er)!
One good thing though is that you can still get a Young Person's Railcard if you are a full-time student. I don't think I fool any of the ticket inspectors though (I'm a young looking 40 but there are limits!)