Sen. Marco Rubio (R-Fla.) speaks at Town Hall Los Angeles on Tuesday. (AP Photo/Jae C. Hong)

LOS ANGELES -- If there was any doubt that Marco Rubio's youthful presence had become a defining feature of his bid for the White House, it was erased here Tuesday when the 43-year-old Republican senator from Florida was introduced at a luncheon as the youngest presidential candidate.

Rubio, chuckling and a bit sheepish, stated for the record that he has a birthday coming up. "I'll actually be 44 in about three weeks," he said, "but I feel 45."

For the most part, though, Rubio is embracing his youth. In his remarks Tuesday at Town Hall Los Angeles, a non-partisan public speakers forum, he warned that the country could not prosper if its leaders were "trapped in yesterday."

"Our nation stands at a hinge moment in our history where we must decide once again, Do we want to embrace the future or be left behind by history?" Rubio said. "And we cannot embrace this future led by people who are trapped in yesterday."

He continued, "Turning the page is what we need to do as a nation and a society."

Sen. Marco Rubio (R-Fla.) is greeted by attendees of Town Hall Los Angeles before his speech Tuesday. (AP Photo/Jae C. Hong)

Rubio has been aggressively making a generational contrast with Hillary Rodham Clinton, a former secretary of state and Democratic presidential front-runner, and with likely Republican opponent Jeb Bush, a former Florida governor and Rubio's one-time protege.

Speaking Tuesday from the City Club of Los Angeles, on the 51st floor of a downtown skyscraper with a sweeping view of the Southland, Rubio delivered his now-familiar stump speech about his family's history as Cuban immigrants and the American Dream.

"We should be proud of our history," he said, "but yesterday is over, and it is never going to come back. Before us now is the exciting opportunity to usher in the greatest chapter in the story of America."

Rubio also fielded policy questions from attendees. Some focused on foreign affairs, such as his opposition to President Obama's efforts to lift the embargo on Cuba and his view of the nuclear threat in North Korea, which he said was governed by "a criminal syndicate."

In particular, he focused on increasing military spending. "We are eviscerating defense spending in this country," he said, predicting that even amid rising threats around the globe, the most modern weapons could soon be developed by China or Russia rather than the United States. He singled out the space program as an area of future investment.

Sen. Marco Rubio (R-Fla.) speaks at a Town Hall Los Angeles event on Tuesday. (AP Photo/Jae C. Hong)

Rubio also talked about domestic policy matters, from his plans to reignite the country's manufacturing economy by loosening regulations to expanding broadband access and overhauling Social Security and Medicare.

Although neither an early primary state nor a general-election swing state, California has long been a magnet for presidential candidates of both parties because of its high concentration of wealthy donors. It is no different for Rubio, who had several fundraising events and meetings lined up Tuesday before flying back to Washington overnight to attend Japanese Prime Minister Shinzo Abe's Wednesday address to Congress.

In response to Rubio's comments, Eric Walker, a spokesman with the Democratic National Committee, sent the following statement: "Marco Rubio says he's a new type of Republican, but all he's done is champion the same failed GOP policies that the American middle class has soundly rejected."