I agree with everyone else who has commented on this thread: it's never too late. I personally believe that the only variables in piano achievement are 1) training, 2) work, and 3) optimal mindset (i.e., good concentration, confidence, tenacity, and other helpful mental factors). If you do indeed have a good teacher, which I define as someone who can get you to resolve your problems, you will be able to improve at any age. I recently saw a movie called the "Sandy Bottom Orchestra" where several music lessons were depicted. The teachers in all cases very self-righteously treated the student like he/she was lost and nearly hopeless: "No, no, no, not like that; that's terrible! You've got to feel the music...." That kind of teaching is utterly incompetent and counterproductive, although judging by this and so many other music-lesson movies, is very common. A good teacher takes responsibility for his students' failures (within reason). If my students are not succeeding, for example, I immediately think, "what's wrong here and what can I do about it?" Without complimenting myself, that is the mark of a good teacher: a can-do attitude, where the teacher embraces all problems that come up as his duty to resolve.
Back to your question, I actually think adults are even better equipped to succeed than youngsters, although I know that is rather pedagogic blasphemy. Adults have more seasoned digital coordination and often better cognitive resources than children, although children follow orders better. Even recalcitrant young ones are reasonably prepared to do as they're told, whereas adults won't move a muscle unless the instructions they're given make perfect sense to them. Thus, we teachers have to be both piano pedagogues and lawyers, making a good case for everything that we know from experience is valid. Contrastingly, we have to be child psychologists when teaching youngsters! Piano teaching and learning is both challenging and exceptionally rewarding no matter what the student's age.
Dr. Steven P. Niles, USC's Thornton School of Music (piano performance)
Instructor of Music, Los Angeles Mission College (LACCD)