We only recently gained the ability to tag people in Apple Photos on iPhone and iPad (the feature is called Faces). Tap any photo that contains faces, then swipe it up. Below the Effects bar (which has some fun stuff) should be a People section.
Tap any person Photos identified, and the next screen will offer more examples of that person. There should be a ‘Review’ option at the top. Tap that, and Photos will allow you to identify more photos to improve its automatic detection.
Bonus points: As of iOS 11 and High Sierra, Faces now sync between all your devices.