A lot of people have a different self-image from what they are. Some people have deep-seated feelings that they are a different gender than what their genitalia might suggest. Many people wish to look youngerNot a,“more normal”, or “more beautiful”. A few wish to look older, have an amputated limb, et cetera.
The reasons behind these feelings are often complex and may be influenced by disease (including mental disease), development, events in their past, personality, the patient’s take on society and social norms, and lots of other factors. For that reason, almost any place that does gender altering surgery would insist on psychological testing. Such testing is probably in the best interest of the patient since it might help determine of their are other issues (e.g. psychosis), if surgery is the best way of addressing the problem, reduce impulsive decisions that the patient might later regret, etc. The best surgeons I know would almost certainly insist on such testing for a case like this.
Not all surgeons are equally ethical. There was a good article in Harper’s magazine in July 2001 (on the net, if you google “dr. daedalus wings”) talks about an eccentric plastic surgeon who dreams of giving people wings. You could argue making someone look younger is not philosophically different from making people look older, but most plastic surgeons do not insist on psychoanalysis before injecting some botox. I would not expect such a cavalier attitude towards looking older, since it is less common and more visibly against social ideals. Some people might find this hypocritical.
The doctor has an ethical obligation to try to prevent harm and maximize benefit to the patient, act in the best interest of their patient, respect patient autonomy, distribute resources fairly and without discrimination, etc. I suspect many surgeons would conclude psychotherapy might be more beneficial than an operation for many of these patients.
With regards to OAP discounts, I am not convinced it would be legally permissible to obtain these, nor would such a person necessarily have acceptable non-fraudulent ID despite their appearance. I personally would not find it morally acceptable, either. If a plastic surgery operation could make someone 46 look 6, should they get into the zoo for free?