Can it be beneficial for a young man to take a multivitamin with 18mg iron if he is worried about being tired often? could the iron hurt him?

No harm: Iron is inefficiently absorbed. Body loss is only a few mg per day, but no where near 18 mg will be absorbed.

Potentially: Taking a multivitamin in general will not harm you. It is unlikely that you will have more energy from it though. In general I do not recommend taking additional iron for men. Women menstruate and lose iron every month, men in general do not lose iron unless they are bleeding. So in general i would not recommend taking iron supplements for men.