Who taught Scott joplin how to play the piano?
Yes, very much so. He had a colourful (no pun intended) upbringing and almost grew up in various Brothels where he learned to play the piano and the new music that was becoming popular. He became known as the father of "Ragtime". The was a movie called "The Scott Joplin Story" which detailed his life from childhood to his death.
John learned to play piano probably in his teens considering his mother played piano and taught him to play the banjo and ukulele. I doubt he learned to read music but Im sure he knew chords when he was a teen. John also taught himself the piano accordion during his teens (which obviously requires piano skills). So the best guess would probably be during his mid to late teens (1956-1958).