When you get into elementary school, they would usually teach you how to play the recorder to start your tastes in music, but another simple instrument you could play is the piano, because there are different levels to it, so it's completely fine if you are a beginner. It's simple to memorize the notes to play, the keys, and reading sheet music gets simpler once you've got the notes down. A good age to start piano at is any young age, because it's like getting a head start the earlier you begin.