Marco Rubio's not the only one in doubt about the age of the universe. However, the level of doubt is a bit tighter in the astrophysicsts; about 4.5 billion is the concensus, but they're working on nailing it down a bit tighter, wanting to get the next digit nailed down.
The unknown seems to be the makeup of the Earth's core. If we know what the core's made up and what the crust we're walking on is made of, the radioactive decay rates will dictate how long ago it formed. We're working on getting to the mantle at present, so the core will be largely out of reach for a long while.