The attack happened in the town of Mubi in Adamawa State as worshipers were gathering for the “fajr” dawn prayer at around 5:20 a.m. local time, said Othman Abubakar, a police spokesman. The bomber was about 17 years old, Abubakar added. Many more people were injured and taken to nearby hospitals. So far no one has claimed responsibility for the attack, but over the past eight years, jihadist group Boko Haram has carried out hundreds of deadly attacks on mosques, schools, markets and churches in northern Nigeria. Boko Haram, whose name translates roughly as “western education is forbidden,” frequently uses children...