It depends:
If you choose to purchase a seat so your infant can travel in a child restraint system (CRS), they need a boarding pass.
If you choose to travel with your infant on your lap (at no additional charge), they don’t need a boarding pass; however, you’ll need a Boarding Verification Document (BVD) for the infant so they can board. If traveling domestically with a Lap Child, you can add them to your reservation. You still have to travel with proof of age.