Jaden Smith and Kylie Jenner in London on March 3, 2013. Credit: PacificCoastNews.com

Jaden Smith has been keeping up with Kylie Jenner! The 14-year-old Karate Kid actor recently began romancing the Keeping Up With the Kardashians star, Us Weekly can exclusively reveal. "They're dating," says a source. "It's new, but they've been friends forever."

The young stars were most recently spotted together on a lunch date at Caffe Nero in London's famous Piccadilly Square on Sunday, March 3. Jaden -- who next appears in the movie After Earth, opposite his famous dad Will Smith -- was in town to celebrate his pal Justin Bieber's 19th birthday.

The March 1 bash at Cirque du Soir was cut short, however, after Bieber's posse got into a scuffle with club security; underage Jaden's presence "caused problems," a source told Us, though Bieber denied those claims via his official Twitter account.

Prior to dating the "Hello" rapper, 15-year-old Kylie -- daughter of Olympian Bruce Jenner and businesswoman Kris Jenner -- was linked to Australian pop star Cody Simpson. She denied the rumors, telling Seventeen in September 2012, "Cody and I are just good friends. We can say that as much as we want and people will still listen to gossip."