Can kids can get brain cancer if they use cell phones so, from what age is it safe for kids to use them?

Any age: The claim about brain cancer makes no sense biologically, the studies are retrospective with weak links (i.e., junk) and the only true-believers are a couple of militants, some lawyers and international-health types wanting to cover themselves. In one of the big studies, people using cell phones got more tumors on the side opposite to the one where they held the phone. Don't trouble yourself.