I mentioned this before. "Around here" the legal age of majority is 16. We'd be letting her vote, get married and inherit property without the need for a trustee, and trying her as an adult if she committed a crime. I'm honestly not sure what the age of majority is in Sweden, but considering that there are countries in which someone of her age is considered to be a legal adult, describing her as a "child" seems perverse.
The reason I should change the legal definition of a word from where I work and live to the definition you use where you work and live is what, exactly ?
Thunberg might be able to walk into a UK pub and order herself a pint but there's no way, no how that she's pulling that off here.