Are there Chinese citizens who are not elderly and don't speak Mandarin?

Having traveled to Xinjiang, watching "To The Wonder" and an interview with young Tajik men speaking fluent (probably native?) Mandarin, I started wondering - are there Chinese citizens of ethnic minorities who can't speak and/or read/write Mandarin?

I suppose, if they're below some age threshold they must have at least basic proficiency in Mandarin due to being through education system. May not be the case for older people, that's why I suspect the elders of ethnic minorities may be the only Chinese citizens who have no proficiency in Mandarin.

People of China, what's it like?