For as long as I can remember, US airlines have had a "10 minute rule" -- you had to present your boarding pass to the gate agent at least 10 minutes before the scheduled departure time, or you could be denied boarding . Frontier now has a 20 minute rule, and I've just seen it enforced! Almost 10 people were denied boarding between 10 and 18 minutes before departure time on a very early PHL flight. The gate agent would not reopen the boarding door. At least one denied passenger was crying. But at the gate across the way, the agent DID reopen the door for "tardy" passengers on a different flight. My recommendation: try not to risk this.