The speaker is standing at the edge of adulthood, around eighteen, and feels that the best part of life is already ending. The refrain "it is only downhill from here" opens every stanza, like a gloomy chant the speaker can't shake. Childhood pleasures such as reading under the covers seem about to give way to paperwork, bills, and schoolwork that never stops.
College looms as both a promise and a threat. It was supposed to be a "diverse paradise," but the speaker has only heard the nightmares: drinking, drugs, date rape, debt, and crushing pressure. Parents offer comfort, the media offers fear, and the speaker seems stuck between the two.
The mood is anxious, restless, and a little self-mocking. The speaker admits they have "slept away" their adolescence, and the ending feels less like hope than a resigned plunge forward into dorm beds, crowded showers, and packed lecture halls.