Since she graduated from Harvard last year, Yehong Zhu has worked for Amazon.com and Twitter in Seattle, San Francisco and London. She’s living in Lisbon now, gathering ideas for her own startup.

In 2014, however, the jet-setting Ms. Zhu was just another teenager from Newnan, Ga. looking for a summer job. And the only place willing to hire her was Waffle House. “Chick Fil-A was too competitive,” she says.

On...