There’s a really good short video (4 minutes) from The Bellman about some of the differences in the demographics between shounen and seinen. https://youtu.be/o0Ts5_q6sMc

TL;DR is that a lot of it is the dialogue complexity. Frieren might appeal to an older audience, but it is written in a simple way that children can understand. Maybe it uses more common words, more furigana, or less kanji. It’s hard to notice, and maybe comes across even less in translated subtitles/dubbings.

Another thing to keep in mind is that anime databases like MAL usually categorize demographics entirely based on the magazine in which the source manga was published. Anime originals and light-novel-source anime don’t have demographic tags. [Though it seems that with consensus/community tags on AniList, this isn’t necessarily true.]
If it was published in a shoujo magazine, it’s technically a shoujo. Even if it’s something like Nana that has themes about contraception, pregnancy, and rape, it’s a shoujo because it was published in Cookie.

I’m personally not really a fan of these demographic categories since they are kinda arbitrary and useless, especially for western anime-watchers who won’t even notice or care if the dialogue of the source manga had more furigana (reading/pronunciation guides) or not.
And as an adult male who accepts shoujo manga as easily the greatest of any of these categories, I think I can also acknowledge that the demographics themselves don’t really apply to me.

But to just literally answer your question, it’s Nana. Like WOW, it’s pretty heavy for something ostensibly oriented towards children. Also featuring drugs, underaged prostitution, and a character who is a porn actress.