Hi nyonyo,
Your idea of retirees entering amateur competitions is an interesting one. I agree, that if you have to prepare repertoire to create an audition tape, etc., it helps to stretch one's abilities. It's also a kind of lifelong learning as well.
I entered a competition when I was much younger. It was held at the New England Conservatory of Music in Boston. Although I did not win, I was selected as one of the three finalists, so was recognized and got to play in the finalists' recital at Boston University. So I got a pretty good sense for the competition scene.
There are some practical hindrances for an older pianist though. For example:
1) Many of us who had no difficulty memorizing years ago now find it difficult to impossible. There are famous examples--Cortot, D'Albert and even Richter read from scores when playing late in life. At competitions, playing from memory is a requirement.
2) As Schnabel used to say, while younger pianists have broad repertoires, older pianists have intensity of depth in their repertoire choices. Why? Because if you live to be 95, life is way too short! So older pianists often tend to specialize in what they really enjoy playing. Competitions, of course, require a traditional balance of Baroque, Viennese Classical, Romantic, and Modern works. When I was young and had to play a 10 or 15 piece program in auditions every year for 10 years, I had no choice but to conform to that. But now I can be myself, so I spend my time mostly on the Neo-Romantic literature now. Unless a piece meets my own inner aesthetic test for beauty, I cannot develop an interest in it, invest time in it, or or become motivated to perform it.
3) Another issue is the entrants. Depending on the screening process for the competition--which may be thorough or not so thorough--you could conceivably have entrants slipping through who are conservatory trained with MMs or DMAs who are not, admittedly, "name" artists with careers for whatever reason, but are, nevertheless, extraordinary pianists. Such a competitor would find an amateur competition easy pickings for the prize. Reminds me of when I played tennis in my younger days. I used to find at tournaments that it was common for Class B players to declare themselves Class C--an easy trophy for them. Is it right? No, but it happens.
4) For retirees, intensive practicing can be problematic if their spouse is not sympathetic. My wife, for example, hates piano, the entire serious piano repertoire, the repetition of practice, plus she has sensitive ears. So to keep the peace, I limit my practice time to when she is out doing errands. For me it would be a huge handicap preparing for a competition. Others may well have a similar issue.
5) Finally, as you attain senior status, your hands don't have quite the same suppleness and dexterity as in years gone by. Osteoarthritis can be a culprit, or tenosynovitis. Things like that can put one at a disadvantage in playing advanced repertoire in competition. One of the best things about being retired is that you can do what you want to do when you wish to do it... or not do it. There is no external pressure anymore. Competitions though are high pressure with some fun too, of course.
BUT... having said all that, nyonyo, I do not want to take away from your idea, because it is still a very good one. And there might well be retirees who could conveniently participate in a local or regional competition without having any of the impediments I mentioned above. Probably a number of them do enter those events.