I learned it when I was ~13/14 and still speak it fluently 10 years later but it is not very useful.
It merely is a nice to have skill but I'd exchange my Japanese anytime for Arabic or Chinese fluency.
If I were to list up the pros I experienced:
- It is a beautiful and emotional language which makes it more fun for me to read in it than in Deutsch. If you are in for the weeb media, it definitely is more enjoyable in original language because whole layers of emotions and meanings get lost in the translations.
- The Japanese internet is a fun place to be, I liked lurking 2chan back in the days and they have some really good YouTubers, I get a lot of music theory wisdom from there for example.
- I guess knowing languages keeps your brain active...? And you can watch Hentai without subs.
But that's it.
Is above worth putting in countless of hours as an adult? I doubt it.