Pop singer and songwriter Meghan Trainor had already devoted most of her life to music when she scored her first hit in 2014 (when she was 20 years old) with a dance-friendly paean to body positivity, “All About That Bass.” Born in December 1993, Trainor was raised on Nantucket Island in Massachusetts, where she began making music when she was just seven, and wrote her first song at the age of 11. Trainor attended high school at the Nauset High School on Cape Cod; there she sang and played trumpet…