Different piano teachers will require various levels of maturity and knowledge before taking on a new student. Some require that a student be able to read and write, some believe students can learn to read music before texts.
My children both started just before their fourth birthdays, one of them had the attention span to excel from the start, the other we decided had started before the child was read and therefore took a few months off before starting lessons again. I personally did not start playing piano until I was a junior in College. At this point I’m fairly certain they will play better than I do in a year or so but it does not stop me from enjoying my time making music.
My personal experience is that the time is never too soon nor too late to try playing.
