If you plan to hire anyone under 18 years old for the summer, you should be thoroughly familiar with the federal Fair Labor Standards Act's child-labor limitations. The U.S. Labor Department enforces these rules strictly and aggressively. Don't count on good intentions and "close enough" to save the day if you get it wrong.

The applicable restrictions depend in part upon the person's age. The employer bears the risk of misjudging how old a minor is. If a person is illegally employed because he or she turns out to be younger than the employer thought, DOL is unlikely to be swayed by the fact that....