This question just popped into my mind during a little practice session at the piano. At what age does a pianist play at their best?. Can you still play very technically demanding repertoire such as La Campanella, Feux Follets, Chopin etudes, etc in your 70s-80s?
This is not a meaningful question. Why? Because are asking for a general answer that applies to everyone and everyone is different. These types of questions are at best useless. You should learn to recognize them and not use them. Seriously. It is a think. There are many questions in our language which are meaningless. They seem meaningful but they are unsound because they logically cannot be resolved. For example, in mathematics, there is the proposition: "Does the set of all sets that do not contain themselves contain itself" seems like an innocent and meaningful question. But it turned the mathematical world on its head. The reason is that if you assume that it does contain itself then you can easily show it can't. If you assume it doesn't contain itself then you can easily show it can. This leads to a contradiction. Your question is not so grand as it has a more basic flaw in it(one could write it out in logical symbols and show that it has several issues).For example: What do you mean by best? What do you mean by great? Why do you want a specific age?If you refine your question in such a way that it becomes meaningful then what you are really asking is "How does age effect a persons ability in piano"? This is still complicated but it can actually be answered somewhat(it's not perfectly defined but we can wring more meaning out of).Your question is a type of question that a young person asks. My question is a type of question an old person may ask(or actually may know the answer).Any old person knows what it was like to be young and what it is like to be old. A young person only knows what it is like to be young... although if they are around a lot of old people they might infer some truths about what it is like being old.In fact, my, my question can be generalized outside of piano playing since why is piano playing specific to adults? E.g., "How does age effect a persons ability in ____" or better: "How does age effect a persons ability".The same is true of all things, Do you think an old gymnast is better than a younger one? Again, what do you mean by better?So you have to refine what you mean by better.But to get past all this I will give you the general idea: As a person ages they gain some things and they lose other things. They gain wrinkles, wisdom/life experiences, diseases, etc. They lose strength, mobility/agility, motivation, etc.Now, these things are actually "curves"(since age is really time). The reason why your question cannot be answered is it is different for everyone. These curves are not even lines but can be parabolas or even somewhat chaotic. Generally speaking they are parabolas. with the extrema different for everyone. Every "great pianist" is going to loose all their ability at some point(when they die(and so technically it is not a parabola)).Some people are able to gain in things that one would expect in youth. E.g., if a young person was very lazy and sickly then they might end up exercising and turning to healthy living later in life which improves their conditions.As you can see, it is not so simple. Life is not simple. Every old person wishes they could have those traits of youth. Think of it as a fruit growing on a fruit plant. The fruit starts out unripened, but then it reaches a certain "perfection" and then it starts to decay(that is a parabola, and arc, much like music such as a sonata form). Now, would every old person want to give up their wisdom to be young? Some would, some wouldn't, depends on the person.All I can tell you is that from an old person to a young person(what I would have wished I would have known) is to learn as much as I possibly could and not worry about the future or any issues. Simply to not waste time not learning. Almost anything is possible and through knowledge one realizes this. Learn everyone one can, specially those things they don't know. Through studying you can learn to answer your own questions precisely as you want. Of course it takes time to learn and if you go too slow you might learn something valuable too late so that it isn't that valuable. But it is always better to know something too late than never at all.The main reason is that what you learn through studying and experience is always more valuable than direct answers to meaningless questions. The reason is that you will learn general truths and these apply to many things. So you get far more "bang for your buck". Everything is connected. So when you learn one thing it will effect how you understand the world. Even if your goal is simply to be a great pianist... do you not think that knowing neurology or physics is important? What about math? Gardening? Chemistry? History? Painting? Knowing these things(not necessarily all of it but having some idea) will surely help you in becoming a better piano player? You may think not... but it's because you haven't learned anything about them or not enough. Again, everything is connected. (what that means only becomes more and more clear over time)Worry less and do more is the axiom of life. Again, this may not mean anything to you... or it might mean a lot. It is also complex because do more of what? watching tv? sleeping? Worry less about what? It is very hard to explain certain things to people who simply do not have the experiences to understand those things. Like talking to someone who doesn't understand the language you speak. This is why children have to be guided, because they do not speak the language of life. They will learn it as they age. If they have people around them that speak it well then the child can learn it more quickly and benefit from it. Remember, no matter how young or old you are.... you are going to get older. That is something you can count on. So you have no choice in the matter. It will also play out how it plays out. You will only know how good or bad it plays out once you get there. You definitely can do things to change the outcome. The outcome is not set in stone in some sense. It might be but pointless to think of it that way.If you want to be really good at piano, play as much piano as you can and when you need breaks learn other things. Of course it's more complicated but you'll have to learn that as you go.