Don't be surprised if your next airplane seatmate has a Spider-Man carry-on bag and elite frequent-flier status.

Millions of children ages 5 to 14 fly alone every year as what the airlines call "UMs," or unaccompanied minors. Many small-fry fliers travel year-round, shuttling between divorced parents on weekends, at Thanksgiving and during the winter school breaks. Tons more tykes take to the skies during the summer for camp or vacation with the grandparents.

Each airline has its own set of rules, rates and programs. Most offer the same basic service, but there are differences.

In general, most airlines will allow children ages 5 to 14 to travel without a parent or a guardian as long as parents fill out paperwork, designate a greeter who has a photo identification, and pay a fee.

Southwest Airlines, which carries more than 400,000 unaccompanied minor a year, does not charge an extra fee. American, which carries an estimated 200,000 a year, charges $100 for each way. US Airways, which boards an estimated 100,000 year, also charges a $100 one-way fee.

Travelers who can't fly Southwest or another fee-free airline might consider putting the money toward a ticket for a friend or relative who might be interested in going along.

Some airlines charge only one fee when two or more children from the same family travel together.

Some airlines will allow kids to fly alone only if they're not making connections. Others will allow minors to take a connecting flight only if they are over a certain age. And in response to a several much-publicized mistakes in which children ended up misplaced or stuck at an airport overnight, most airlines will not allow the children to be booked on the last flight of the day or on an overnight flight.

All programs promise to make sure the children get on the plane and are delivered to a designated greeter at the other end.

Other airlines offer a few extras. Northwest spokesman Vin Parker says that in addition to supervising children in-flight, the airline has WorldKids clubs in several hubs, including Detroit, Memphis and Minneapolis-St. Paul, that provide fun, secure areas. "The clubs have books, games, videos, toys and complimentary phones that kids can use to call parents/guardians."

JetBlue spokeswoman Allison Eshelman says children usually are seated in the front of the plane. "We show them how to work the TV and/or XM Satellite Radio, and make sure they get all the snacks and beverages they want." The crew also shows children around the plane, and when time permits, the pilots will show them the flight deck and offer them JetBlue wings.

On US Airways, which recently started charging all coach passengers for soft drinks and water, unaccompanied minors will continue to receive complimentary beverages.

Most international airlines have unaccompanied-minors programs and their own set of perks. Air France Planet Blue (PlanEte Bleue) program has a special area for ages 4 to 11 in Paris' Charles de Gaulle and Orly airports. On flights longer than 2 hours, the airline gives kids special meals, games and gifts. British Airways' Skyflyer Solo program will not accept children on itineraries with a night stop or transfers/stopovers longer than six hours, and it provides an in-flight escort service when a number of unaccompanied children are traveling on the same flight. On flights operated by Virgin Atlantic, the services of an "auntie" or a "registered child-minder" can be arranged.

The Department of Transportation, which received 40 unaccompanied-minor complaints in 2007 and 32 so far this year, has a downloadable "When Kids Fly Alone" brochure available at http://airconsumer.ost.dot.gov/publications/KidsAlone.pdf. Airline websites also offer tips for parents.