Schubert composed 'Gretchen' at age 17, and Mendelssohn composed the 'Octet' at age 16. At what age did your favorite composer create their first true masterpiece?"

I'm fascinated by how young some composers were when they created their first masterpieces. For instance, Schubert composed "Gretchen am Spinnradee" at 17, and Mendelssohn wrote the "Octet" at 16.

I'm curious to know: at what age did your favorite composer create their first true masterpiece?