The speaker opens the poem by telling readers how "it"—that is, turning 10—makes him feel.
The first line uses enjambment to play with readers' expectations, however, not actually revealing how this occasion makes the speaker feel until the next line. There's thus a moment of anticipation at the top of the poem, when one might think that what will follow will be some sort of positive feeling. But in the very next line, the speaker punctures any positivity with the revelation that, upon turning 10, he feels like he's "coming down with something."
This simile kicks off the speaker's melodramatic take on growing up. He treats getting one year older as if it were some kind of disease, one he cannot quite name. The anadiplosis of "something" (which appears at the end of line 2 and then repeats at the start of line 3) reinforces this sense that the speaker lacks the language to properly identify what sickness he's suffering from.
While he cannot say exactly what it is, the speaker can compare this feeling to other ailments he has experienced—stomach aches, or "the headaches I get from reading in bad light." A stomach ache in particular is a classic childhood ailment, which reminds readers that the person lamenting the loss of childhood right now is, indeed, still very much a child!
This a free verse poem, meaning it has no meter or rhyme scheme; it sounds conversational and casual. Despite their straightforward language, however, these lines are packed with consonance and assonance (as is the whole poem). Note, for example, the sharp "/k/" sounds of "makes" and "coming," as well as the round sounds of "coming," "something," "stomach" and "from."
There's also plenty of sibilance here, which might evoke a kind of childish lisp. Take lines 2-3:
[...] something,
something worse than any stomach ache
Even though the language of the poem is kept loose and conversational, it still has some subtle music.