[throwaway99111]

This seems like a very common sentiment regarding anime especially among white people in America. I think the reason for that is partly stylistic choices of anime but if we are really honest, Japanese people if not many Asian peoples appear as children to white people. This of course seems mostly visual/aesthetically based, a similar but opposite disposition towards black people exists in America[0]. Japanese animators draw characters that emulate characteristics they find familiar, which makes sense, and thus they appear "childish" to Americans.

I could bring up old racial stereotypes regarding Asian people to further bolster this point but I think most people are aware of this.

[0] https://www.apa.org/news/press/releases/2014/03/black-boys-o...