Estimating causal effect for educated people on wages
Yes, you have to worry about the difference between correlation and causality. In these situations, it helps to try to force a case of ommited variable  bias.

In this case "talent" or "effort" is unobserved. Both this might make people more likely to pick up additional education, and also be more successful at their jobs.

Or family background: If your parents are high achievers, they might induce you to take more education, and they also are well-linked to ensure you get a well-paid job.