The only fc who is actually a teenager with resources is Adachi Yuto, although whether he looks as young as those two is kind of questionable but he’s still 18 anyway so!!
Anyway, here’s some more fcs. I just moved a bunch of fcs from the teen to 20 section on my masterlist so they could probably work. Most of these do not have resources, because they aren’t as well known, but tbh if you message me off anon then I’d be happy to make some if I can.
- Maeda Mackenyu (20)
- Nakagawa Taishi (18)
- Kitamura Takumi (19)
- Watanabe Tsurugi (20)
- Yokohama Ryusei (20)
- Sano Hayato (18)
- Yamada Ryosuke (23) has a tonne of resources and some from younger roles too
- Shison Jun (21) he can pass for 18 but not really in comparison to those mentioned
You can also just take a look here and pick someone, I suppose. Sorry I could not help much, but teenage Japanese fcs really have next to no resources especially with what you’re expecting from the resources.
Also, please do not use a much older (25+) fc who you think looks younger to play a teenager.