低龄 is an attributive adjective meaning 'young in age' or 'low-age'. It describes people (especially children or teenagers) whose age is lower than a typical or expected threshold for a given context — for example, 低龄儿童 ('young children'), 低龄留学 ('studying abroad at a young age'). It is NOT a direct synonym for 'underage' (未成年), which has a legal connotation. Use 低龄 when you want to emphasize that someone is younger than the norm for an activity or category.