As a general rule, children under 18 don't have to show identification at all when passing through Transportation Security Administration checkpoints with an adult. But airlines can impose their own identification requirements, so your kids might need to show proof of age at the check-in counter or boarding gate, especially if they look older or younger than they actually are. If your children are traveling as unaccompanied minors, they're protected by a different, rigorous set of guidelines designed to combat trafficking and other hazards.

Children Traveling With You

Most airlines allow children younger than 2 years of age to travel as lap infants, which means exactly what it sounds like: The little one doesn't have his own seat, but instead makes the entire trip sitting on your lap. This is a classic instance when airlines may ask you for proof of your child's age, especially if he looks older than his actual age. A copy of his birth certificate paired with your own ID does the job, as does a passport (if he has one). Older children are required to have their own seat, but may still be asked to show proof of age, and may even be asked questions to help prove their relationship to you.

Age Limits for Unaccompanied Minors

Each airline also imposes its own rules for travel by unaccompanied minors. As a general rule, children under 5 years of age absolutely must travel with an adult, and children 15 and up have the option of traveling without any additional safeguards or unaccompanied minor procedures, although those safeguards and procedures are still available, on an optional basis, for anyone under 18. Some airlines allow children as young as 12 to travel without unaccompanied minor procedures.

Booking Flights for Unattended Minors

Pay close attention when you book a flight for an unattended minor. Not only will you probably pay an "unattended minor fee," which pays for the time and care given by airport or airline staff, but you may also be restricted to only a certain subset of flights. Again, each airline's rules differ, but many restrict younger children to traveling on nonstop flights. Most will not let unaccompanied minors travel on the last flight of the day (unless it's the only option), and some won't allow unaccompanied minors to travel on flights that begin late at night. It's best to call your airline and have a customer service agent walk you through the options for your young, solo flier.

ID for Pickup/Drop-Off

Your unaccompanied minor usually won't need to provide photo ID, but you will have to show your ID when you drop him off at the airport, and so will the person picking him up. Be prepared to leave your address and phone number, plus that of the person picking up your child, and bring a copy of your child's birth certificate as proof of his age. Some airlines will also provide a badge at this point, which becomes the only ID your child is required to show – but he must keep it on his person through the entirety of the flight.

Minors on Their Own at the Airport

Your job isn't necessarily done once you get your child to the airport. Your airline may issue a gate pass so you can escort them all the way through security to the boarding gate. In fact, they may insist on it. Plan to be available until you have confirmation that your child's flight has left the ground, and keep in mind that if your child's flights are for any reason delayed or canceled, the airline may contact you for permission to rebook.