The phrase "sweet sixteen" connotes youth, vibrancy, and innocence. However, there are numerous antonyms or opposite words that can be associated with this expression. Instead of sweet, you can think of harsh, bitter, sour, pungent, or acrid. Alternatively, you can replace the word "sixteen" with its corresponding antonyms, such as old, ancient, elderly, mature, or over-the-hill, which imply a lack of youthful energy and vitality. Using these antonyms, you can create contrasting images that suggest age, bitterness, or negativity to contrast with the sweetness and optimism of "sweet sixteen.