The Twilight Saga stars were spotted toasting the actor's special milestone with friends, including actress Ashley Benson and Arnold Schwarzenegger's son Patrick at a low-key bash in a Los Angeles bowling alley on Saturday night (February 9, 2013).
The former co-stars snacked on pizza, sliders, chips and guacamole, and listened to music as they bowled together in a private area at Lucky Strike.
A source told USMagazine.com that Kristen, 22, arrived later than the most of the other guests and showed up after midnight with a small group of friends, wearing a backwards cap, ripped T-shirt, leggings, and Converse shoes.
She was reportedly being "really sweet" to the birthday boy and joined in as the group sang him "Happy Birthday" before leaving at around 1.30am.
The eyewitness said: "She was being fun and social, bowling and high-fiving. She and Taylor definitely seemed to be good friends and she was living it up."
But Taylor, who turned 21 on Monday (February 11, 2013), was on his best behaviour at the bash and "drank nothing."
The insider said: "Taylor was super nice, very polite. Everyone was having a blast. Everyone was super into the bowling."
Kristen's on/off boyfriend Robert Pattinson, 26, was not present.
The pair reportedly agreed to a trial separation while the British actor films The Rover in Australia, and despite agreeing to no contact while he is away, the actress—who cheated on him with married director Rupert Sanders last summer—recently wrote him a lengthy love letter in a bid to repair their fractured relationship.