It's not the 1000-year-old-who-looks-8 trope, it's the 1000-year-old-who-looks-25 trope. She's never portrayed as anything close to childlike.

Not really, she really is just an old elf lady who looks like a young adult human. There's nothing sexualised about her, and she's not childlike. For example here she is next to a human child who becomes her apprentice:

And here she is talking to a human priest she was adventuring with 50 years ago, he's grown old, she hasn't changed:

And here she is head patting him, to give him reassurance that he did well after he confides that he never felt like he figured out how to be "a grown up" and just kept faking it while growing old:

The human child is Fern, and the old Priest Heiter picked her up after she was orphaned by a demon attack. Heiter asks Frieren to keep taking care of Fern after his death, and mentor her in magic.

It's a really nice relationship driven story with an interesting storyline and magic system.

As everyone else has said, she's not a child-but-actually-ancient. The show is not about sex. It's about mortality. I started watching it, and I want to continue, but it's really depressing. I wasn't in the right mood to keep going.

As far as I'm aware, there are absolutely no sexual undertones in the show. It's sometimes funny, but it's also pretty serious.

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.