US Senator Marco Rubio launched his presidential campaign on Monday at a rally in Miami, calling for a new era of American leadership that is not "stuck in the 20th century."The conservative 43-year-old first-term lawmaker, son to poor immigrants from Cuba, presented the 2016 presidential election as "a generational choice" - a clear jab at his more senior rivals, notably the newly-announced Democratic candidate Hillary Clinton and his fellow Republican and potential adversary Jeb Bush.