Quick tip: Documents needed when flying with a lap child

Editor's note: This is a recurring post, regularly updated with new information and offers.

Although my family is now out of the lap infant phase, having my children fly with me for free up until they were 2 years old was my favorite way to save money (and points) on our family vacations. I felt comfortable with this decision, although I know that it is not for everyone for a variety of reasons.

Since we typically fly Southwest (hello, Companion Pass!), there was almost always an empty seat for my son and daughter when they were lap infants. (If you're new to flying with an infant, check out TPG's flying with a baby checklist and how to plan award travel with an infant or lap child.)

While most frequent flyer parents know that the magic age range to fly as a lap child is from birth until the day before they hit their second birthday, what some may not know is that all U.S.-based airlines — and most likely international airlines as well — technically require you to prove your child's birthday. Of course, many airlines will not ask for documentation on any given trip but if they do, you want to be prepared. Not having the proper documentation could require you to purchase a seat for your infant and same-day ticket prices can be quite astronomical (assuming there is even still availability on your flight).

Note that if you are traveling internationally, your infant must also have his or her own passport — whether they are a lap infant or are in their own purchased seat.

Documentation required for lap infants

To prove your child's age to an airline, one of the following documents will suffice:

• Child's passport (how to get a U.S. passport for an infant and things to know about children's passports)
• Child's birth certificate
• Child's immunization form or other medical records may also work domestically

When my children were in the lap infant age range, I always kept a copy of their birth certificates in the front pocket of both my and my husband's suitcase. This way, we always had it with us to show the ticket agent. Keeping an electronic copy on your smartphone isn't a bad idea, either.

Will the airline actually ask for documentation?

In the United States, nine out of 10 times, you won't be asked for proof of age on most airlines when flying with a lap infant. With that being said, there is a very specific exception to that general rule. Southwest Airlines almost always asks for proof — even if your child clearly looks under 2. I was even asked by a Southwest agent for proof when my son was just a 10-pound baby. It is Southwest's policy to check for age verification on lap infants regardless of appearance, so be prepared with an appropriate age-verification document.

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Friends or acquaintances may tell you that age verification is not required for your lap child, but that is absolutely not true. While children don't need an ID for TSA, to fly as a lap infant and document to the airline that they are indeed under 2 years old, you need to be prepared. If you actually purchased a seat for your child, no identification is required since you don't need to prove they are under 2.

Related: Surviving a flight with children of every age

Bottom line

Unless you are flying on Southwest Airlines, the chances of being asked for some sort of identification for your lap infant on a domestic flight is slim (especially if they still look like a baby), but you are better safe than sorry. As always, in the realm of family travel, you want to be over-prepared with documents, not under-prepared.

Have you ever flown with a lap child? What was your experience with documenting their age?