Frankie, often referred to as the "Bonus Jonas," is 18 now and has probably had more of a career than you realised. He produces music under the name Mephisto on his Soundcloud and shares photos of guitars on Instagram, so it sounds like music is a part of his life, just like it is for his brothers.
Frankie also dabbled in acting, including having a part in the TV show Jonas with his brothers. His biggest role, though, was as the voice of Sosuke in the film Ponyo.