Final answer:
The word 'oldest' in the sentence 'John Stuart Mill was an oldest son' is an adjective.
Step-by-step explanation:
In the sentence, the word 'oldest' is being used to describe the noun 'son,' so it is an adjective. Parts of speech categorize words based on their function and role in a sentence. In this case, 'oldest' is providing information about the son, describing his age.
Learn more about Parts of speech