An 11 year old British girl composed a 2.5 hours long German-sung opera (a reimagined Cinderella) which just showed in Vienna, and received a standing ovation of 10 minutes. She plays the piano and violin, and actually accompanied the singer playing Cinderella on stage with her violin.
A shorter version of this work was shown in Israel last year, so she must have been pretty young when she first started composing it.
What a wonderful little person!
http://www.bbc.com/news/world-europe-38467218