Here's what the Southwest site says:
"To use Online Checkin @ southwest.com and print your Boarding Pass, you must:
Be age verified if traveling on Senior, Youth or Child, or Infant* fares.
Check in beginning 24 hours prior to your scheduled departure
Check in at least one hour prior to your scheduled departure
Have a Ticketless reservation
Be checking in for flights that have not already departed
Have purchased an eligible fare type (Customers holding reservations for group or military fares, and those requiring an additional seat must proceed to the airport for checkin)
*This only applies to infants traveling on an Infant fare. Children under 2 who are riding as lapchildren must still present a valid birth certificate and checkin with a Customer Service Agent at the airport.