After a long day of filming on the NYC set of her new movie, The Baster, on March 22, Jennifer Aniston ducked into the Tribeca Grill for dinner. "She was so sweet," says an onlooker. "She was in a good mood." But was Jen, who turned 40 on Feb. 11, just putting on a brave face? "She hasn't been the easiest on set," an insider working on The Baster tells Life & Style. "She's being controlling and snapping quite a lot. It's pretty out of character for her."

Jen's beef? She's having a problem with the fact that her character, who's trying to get pregnant, ages from 40 to 48 when the movie jumps ahead in time. "She keeps saying it's not believable because she looks much younger," says the insider. "She asked the makeup artist what high-tech effects they're going to use to age her, and she was disappointed to learn there aren't any."

For more on Jen's fear of looking old, pick up the latest issue of Life & Style, on newsstands now!

Photo Credit: Retna