“妙龄的”常见英文表达可以是 “of a tender/young age” （侧重于表达年轻、稚嫩的年龄段） ，不过更自然贴合“妙龄”（常用来形容年轻女子处于青春美好、适婚等美好年纪）这种语境的常用表达还有 “in one's prime of youth” 或更简洁常用的 “young and in the prime of life” （强调处于青春美好的鼎盛时期） 。
例句：
She was a young lady in her prime of youth, full of vitality and charm.（她是一位妙龄女子，充满活力与魅力。）
The actress was in the prime of her youth when she starred in that famous movie.（那位女演员出演那部著名电影时正值妙龄。）