Here's why:
* Der Junge: This is the nominative case of the masculine noun "Junge" (boy) and is the subject of the sentence.
* ist: This is the present tense of the verb "sein" (to be).
* rothaarig: This is an adjective meaning "red-haired" and correctly describes the subject.
The sentence follows the standard word order for German declarative sentences: Subject-Verb-Object/Adjective.