妙龄 refers specifically to a girl's late adolescence — roughly ages 15 to 20 — when she is at her most youthful and attractive. It is a noun but almost always used attributively before another noun, most commonly 妙龄少女 ('a young girl in her prime'). The English gloss 'late adolescent girl' can mislead learners into thinking it means 'girl' itself; in fact, 妙龄 describes the age period, not the person. You would say 她正值妙龄 ('she is in the bloom of youth'), not 她是一个妙龄.