In the 21st century as in other centuries words can reflect the politics of those who use those words as an influence, but sometimes the meaning doesn't reflect how the word will be used.
To define the word "adult" is to say it is a noun, adjective & a verb. Wikipedia actually goes into full detail about what the word "adult" means insofar as the legal terms and more.
One of my research sources defines the word "adult" as a plant in addition to other criteria.
However, the word "child" is solely defined as a noun, the word "child" becomes an adjective when the word is changed to "childless." One source noted that the word "child" could be a reference to a product. Another research source mentioned that a "child" is a son or daughter of any age, but, there is another research source that defines the word "child" as a "human offspring," and one more research source did note specifically: a child is immature and childish.
Let's assess from the "child" prospective: the ultimate objective is to be an "adult" because the adults are in charge.
If consideration about the number of parts of speech, such as "adverb"/noun/adjective, are attributed to the world "adult," then the importance of this word "adult" is shown by the number of speech parts. Consider all the "adult" speech parts, versus the "child," the winner is the "adult," since "child" is solely a noun. And I think it might be fair to say that one can make a conclusion of sorts about the word "adult" versus the word "child.
The why, is self evident.
A child isn't in charge, therefore, the assumption is that an "adult" is superior to a "child," but, so far in this 21st Century of innovations, there is no movement to effect the legal standard for a "child" to be in control of the decisions that are made by "adults" on their behalf.