That trope bothers me too, but basically any anime female is going to look the same anywhere from 12 to 40 due to the way the features are stylized. So, looking for context clues is how you find the age of a character if it’s not explicitly said. Frieren is absolutely treated like, dresses like, and acts like an adult. She’s smaller than a human man, larger than a dwarf or human child, and while she’s infantalized a little by Fern it’s very much in the “how are you this old and still not taking responsibility for x” way.
That trope bothers me too, but basically any anime female is going to look the same anywhere from 12 to 40 due to the way the features are stylized. So, looking for context clues is how you find the age of a character if it’s not explicitly said. Frieren is absolutely treated like, dresses like, and acts like an adult. She’s smaller than a human man, larger than a dwarf or human child, and while she’s infantalized a little by Fern it’s very much in the “how are you this old and still not taking responsibility for x” way.