This might also help: when a verb is conjugated in English, usually the only change occurs with the third person singular (I run, you run, she runs, we run, you run, they run). In Italian (and Spanish, French, Portuguese...) this is not the case, there are 6 different conjugations, so dropping the subject pronoun is not a big deal because the conjugation of the verb still indicates who did what!
My understanding of Italian has always been that where there is ambiguity from the omission of the pronoun but one option is more likely than the other you should always assuming the most probable interpretation. Because if it were the less likely possibility, the speaker or writer would have specified. ie. In this case (in the lack of any context or clues to indicate that the intended subject is loro), you should assume the speaker means "I am a boy" not "They are a boy".
I'm not a native speaker though, so if any native speakers want to comment on that, that'd be cool.