The boy is not ______ - he is ______ - and therefore, should not be tried in court as an adult.
mature, juvenile
The first blank should be filled with a synonym for adult as the statement suggests that he should not be tried because is not an adult. Hence, options with first blank containing forms of youth can be eliminated. From the remaining options, puerile means immature and can be eliminated. The second blank should be filled with a form or a word similar in meaning to youth. But inveterate means established and hence can be eliminated. Hence, correct option is mature and juvenile.