My faves in the "mature" category would be Aki Sasaki (39), Yui Hatano (31), Hibiki Otsuki (31), Tsubomi (31) and Yu Kawakami (36)
Honestly I'm the same age as Yui, Hibiki, and Tsubomi. Personally I wouldn't consider any of them "mature" actresses, but according to the industry they are labeled...