The Defense Department and a private contractor have been building an extensive database of 30 million 16-to-25-year-olds, combining names with Social Security numbers, grade-point averages, e-mail addresses and phone numbers. The department began building the database three years ago, but military officials filed a notice announcing plans for it only last month. That is apparently a violation of the federal Privacy Act, which requires that government agencies accept public comment before new records systems are created. David S. C. Chu, the under secretary of defense for personnel and readiness, acknowledged yesterday that the database had been in the works since...