It is possible that one factor modifies the effect of one factor on another (effect modification). For example, it could be that there is a strong association between two factors at younger ages, but at older ages the association could disappear. Age is modifying the association between the two factors of interest. Therefore, examining the relationship between two factors is not straightforward, and can be further complicated by confounding and interaction.
Also see: Bias, Causality, Confounding, Confidence Intervals, Interaction and Small Numbers.