It is really a pity that most of us seem to live in a materialistic society where "success" is only counted in terms of money and grade of fame. As not everybody can be equally successful when you have that perspective, you also implement a most unnecessary competitive factor in it, where you have to "defeat" your competitors in order to get into that school, top that list, win that competition and sell those tickets.
If you are an ambitious teen piano student, you might be so affected by this perspective that you simply DON'T BELIEVE older students (say, +50 years) when they claim that they are not interested in that kind of "success", they play out of other reasons and have other goals. You think "that is what they are saying to mask the truth, which is that they CANNOT reach up to that level." You cannot get rid of the idea that "that level" simply is the highest and most desirable, that we are ... well, competing about the position on top of that mountain. And that playing as a hobby is another way of saying "to kill some spare time".
But one advantage of getting older is that you - or most of us - shift that perspective. We realize, eventually, that this kind of life, the top-concert-pianist-life, is not very glamorous and fun as such, not worth the price. And that "success" could mean a lot of things. I would say that a piano teacher who inspires lots and lots of young and old students and arranges many kinds of nice activites connected to piano playing, is incredibly successful. So is the senior who plays for his own pleasure only and constantly experience the joy and thrill of taking on challenges and learn new things, which makes him feel young and curious again. So is the worn-out housewife who finally has found a private sanctuary, away from family duties and the "serving-someone-else" syndrome. Because these people find a deep sense of meaning and personal achievements in their life and THAT, my friends, is the true meaning of the word "success".
We are all unique, and we are all born with a desire to express ourselves, our uniqueness. As pianists we must also be allowed to be unique and have our personal goals. A personal thing for me, for example, is to play the Appassionata, as I adore this piano work. Nobody will probably ever pay money to hear my version of it, and frankly I don't care. I just love working with it, to explore all the new depths in it while I work with bar after bar, constantly seeking new ways to conquer, learn and improve. I have the time of my life! What else matters? So, my goals are about learning and playing some of the music I love, not to impress an audience and get "famous". I don't like performing.
It is a pity that we so often deny people to reach their goals and enjoy their own successes, with stupid comments like "at that age, you are too old", making them finding excuses to diminish themselves. "Oh, I have no ambitions, I just dabble a bit, you know". "I am not very talented, really, I just want to have a little hobby."
I want to tell all these people that YES, you really have the right to be as ambitious as you please, that you don't have to make up excuses for "not being that good" and you are NOT too old. Forget that sh*t about brain plasticity, please. It has been proven it is not very true at all. What matters is your own attitude, your own beliefs about your capability. And attitudes are not childish wish-lists to Santa, they are based on knowledge, experience and considerations that could be rather complex. So just saying "I want to, I want to" is not enough to make you succeed in your ambitions. It goes much deeper than that.
But ... if you still get the idea at the age of 57 that you want to learn to play the piano and pursue a career as a concert pianist, then? Is it possible? I say yeah, it is. You just have to pay the price and do the work that is needed. So far I have not seen anyone willing to do that, so the question is hypothetical, therefore irrelevant. Let's rephrase it: is it possible to reach your personal goal in piano playing even if you are not very young anymore?
Of course! And don't be "modest", just do what you wanna do. As long as you are willing to learn, you are able to learn.