When searching for flights for a passenger who is under 18, you should now specify their
age instead of a
type. For passengers over 18, you can continue using
"age": "adult" as before.
Different airlines have different definitions of who is an "adult", "infant" and "child" which can have a big impact on pricing. Some even have extra categories (for example "young adults" on British Airways, who pay lower taxes).
For some inspiration on how to handle this in your UI, check out Skyscanner: