This was harder to answer than I thought, pushing up to 20 eliminates a lot of contenders I think. I would say maybe Vinland Saga? The main character starts as a child, but it also covers a large time span so he hits twenty partway through the second season. Major recurring characters are almost all adults throughout. Its a semi-historical anime set mostly in England and Denmark during a war between the two. It’s a show that deals heavily with violence and trauma, has some very complex characters, and I can’t recommend it strongly enough

I’m so glad you enjoyed it! The series blew me away and it’s wonderful to see others get to experience it. Thank you for the update 😃

No mecha, but for War is Bad and child soldiers I feel like you can’t do better than Vinland Saga. Its a story about war, the trauma it causes, and recovering from that trauma. Its a historical war manga by someone who abhors violence and its stunningly beautiful