Paul tells NBC's "Today" show his candidacy is a response to "endless, undeclared, unwinnable wars dumped on the young people" and a soaring national debt.
The 75-year-old congressman says he doesn't believe he's too old to be president, saying he thinks young.
Paul won a GOP straw poll at a weekend Republican conference, garnering 612 votes, far ahead of former Utah governor Jon Huntsman, who got 382 votes. Huntsman is expected to get into the race later this week.
Paul says the U.S. "shouldn't be warmongers. We shouldn't be the policemen of the world."