You are investigating the role of physical activity in heart disease, and your data suggests a protective effect. While presenting your findings, a colleague asks whether you have thought about confounders, such as age. Under which condition will age be considered a confounder while interpreting your data?

1. It is associated with physical activity but not heart disease
2. It is a risk factor associated with physical activity and heart disease
3. It is part of the causal pathway by which physical activity affects heart disease
4. It is a risk factor for heart disease but not associated with physical activity