Characters under the age of 18 are frequently treated as adults in-universe. Descriptions of the main character are provided with intent to arouse the reader before he is 18.
This happens very late in the novel (as of 2023, it's part of the machine translation); a prominent antagonist is crushed semi-graphically between two stone doors as they close.
There is extremely graphic onscreen child death of a VERY young child (approx. 3 years old); I would recommend skipping pages 245-246 of the official English translation for vol 2 if this would trigger or upset you; you won't miss any plot points. There is also a semi-graphic child death (of a teenage girl) in the Tianyin Pavilion arc (currently mtl).
YWX can potentially be considered a trans male character (the closest literary parallel I can come up with would be Mulan or Alanna of Trebond), but is treated as female by the narrative and all characters after being outed as AFAB.