MAIDUGURI, Nigeria — Islamic extremists in northeast Nigeria are turning their guns on elderly people, killing more than 50 in the past week in a new tactic that has instilled more fear in areas the militants call an Islamic caliphate. Residents from five villages say people too elderly to flee the Gwoza local government area are being rounded up and taken to schools where the Boko Haram militants open fire on them. “What they are doing now is to assemble the aged people — both men and women ...