What is a common sign of beer that has gone bad due to aging?

A cardboard flavor notes is a common indication of beer that has deteriorated due to aging. This specific off-flavor is typically the result of oxidation, which can occur as beer is exposed to air over time. When beer oxidizes, it can develop stale flavors and aromas reminiscent of cardboard or paper, which detracts from the original freshness and complexity that the brewer intended.

In terms of the other options, a strong hoppy aroma would indicate a fresh beer or one that has been recently brewed, especially in styles that emphasize hop character. Excessive carbonation can indicate issues related to fermentation or packaging but is not a consistent sign of aging. Similarly, a sweet aftertaste could be associated with certain styles of beer or residual sugars, but is not indicative of beer going bad due to aging. In contrast, cardboard flavor notes are a clear sign that the beer has lost its freshness and has suffered from time and exposure to oxygen.