Under-aged is a term typically used to describe individuals who are not yet of legal age to engage in certain activities. Here are a few synonyms that can be used in place of "under-aged": underage, juvenile, adolescent, minor, youth, teen, young person. These terms all connote a lack of legal maturity or the age of majority. In some contexts, the preferred term may depend on the activity being discussed; for example, "minor" may be used in reference to legal matters involving contracts, while "juvenile" may be used in the context of criminal justice for young offenders. Ultimately, the appropriate word choice will depend on the specific context and the desired connotations.