30 Under 30 Music: Shawn Mendes, Fetty Wap, Halsey And The Class Of 2016

Masters of the Universe: The 30 Under 30 gatefold, featuring musicians Shawn Mendes and A$AP Rocky. (Photo: Jamel Toppin).

For most teenagers, summer job prospects are limited to mowing lawns, babysitting or working a cash register. Not so for Shawn Mendes: the 17-year-old singer-songwriter spent the dog days on tour with Taylor Swift.

“The way I keep myself sane is by thinking of it as fun”–and not a job–says Mendes, who parlayed Vine stardom into a major label deal and a chart-topping debut...