Jennifer Aniston has excellent behind, states fitness mentor: An American fitness trainer has said that former 'Friends' star Jennifer Aniston has got the best behind. Jackie Warner, 42, creator of the new Crunch-Free Xtreme Abs dvd, spoke of 41-year-old Aniston's derriere. "She's always wearing those little short dresses and that butt just goes boom," Us magazine quoted her as saying. "I guarantee she's in the gym and she's doing a lot more than Pilates," she added.