Identify one risk factor for gallstone formation.

One significant risk factor for gallstone formation is age, particularly in individuals in their 40s. As people age, the liver's cholesterol output and the gallbladder's ability to concentrate bile can change, contributing to higher rates of cholesterol supersaturation in bile, which is a key factor in gallstone development.

Additionally, the risk of gallstones increases with age due to hormonal changes and other metabolic factors, making individuals in their 40s more susceptible. While gender and diet can also play roles in gallstone development, age is a well-documented and widely recognized risk factor that reflects a physiological change over time.

The other options mention factors that can influence gallstone formation but do not comprehensively define age as a specific risk. Men can develop gallstones, but women are more affected, particularly during childbearing years. A poor diet can contribute to gallstone risk, but it is more often combined with other factors rather than being a standalone risk like advancing age. Thus, focusing on age as a singular risk factor highlights its importance in the context of gallstone formation.