Assessing methodological errors in a study

Suppose the research question seeks to see if there is a causal effect between exposure A and an outcome B among 18-24 year olds in a particular neighbourhood. However, a cohort study was conducted that included 18-40 year olds in the study population.

Could I say that the study sample is non-representative of the target population? In that case, what kind of error is it and how would that affect the results (such as odds ratio) compared to the truth?