(CNN) -- Sometimes it's necessary to send Junior packing -- to see far-off relatives or a cross-continental parent. In general, if your kids are under 5, they aren't going anywhere on a plane without you or a guardian. But if they're older, they can travel alone, under certain conditions.

The obvious: infants must be accompanied by an adult, as is the requirement for most children under 5. One infant (usually defined as a child under 2 not occupying a seat) per paying adult age 12 or over may travel free. Some airlines (in some cases) have reduced-fare seats available for infants. Children 2 and over must pay full adult fare.

Though children under 2 may fly free, they must sit on a guardian's lap if the plane is full. The FAA and the National Transportation Safety Board have found that children are much safer flying in an approved safety seat (you have to bring your own) than they are on a parent's lap. Even with discounts, a fare for a child may seem extravagant, but holding a child on your lap in a plane is no different than doing so in the front seat of a car -- it's an unnecessary risk that can have fatal consequences.

As for older children, rules and regs applying to flying alone aboard U.S. aircraft vary from airline to airline, but not by much. The following are general guidelines -- please ask your airline before booking flights for unaccompanied kids.