Yes, old zucchini can get bitter as it ages. This bitterness is caused by an increase in the concentration of cucurbitacins, which are naturally occurring compounds found in the zucchini plant. When these compounds become too concentrated, they give the zucchini a bitter flavor. To avoid this bitterness, it's best to use zucchini when it's young and fresh. Look for small to medium sized zucchinis that have a bright green color and a glossy skin. Avoid any that have soft spots or are overly large, as these may be too old and bitter. Additionally, make sure to store your zucchini properly so that it doesn't spoil quickly. Wrap it tightly in plastic wrap and keep it in the refrigerator for up to one week.