This post was updated at 6:15 p.m. ET
Readiness to be president is a threshold question for many candidates. That's especially true when that candidate is 43 years old and a freshman senator.
No, not Barack Obama, but Marco Rubio, the Republican senator from Florida, who announced Monday that he's running for president.
"I'm certainly capable from Day 1," Rubio told NPR's Steve Inskeep in an interview in Miami hours before he announced. "I'm very confident that I have the capability from Day No. 1 to lead this country."