School-age is the time in a child's life when they are old enough to go to school. Countries set this by law, so the exact years are not the same everywhere, but the idea is the same: not a baby any more, not yet out at work — the years of sitting in class. You will hear it most as a describing word before another noun: school-age children, a school-age daughter. You can also say a child of school age. It is ordinary, slightly official language — the kind you read in the news or hear from a school or a doctor — not slang.