Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
Fan Xian's strength was around the eighth rank. Although he had won the battle with Cheng Jushu easily, he had not surpassed the level of the Eighth Stage. In the battle before Fan Xian and Teng Zijing joined forces to deal with Cheng Jushu, his strength had increased. He also said that after that battle, his zhenqi had gone up another level. Fan Xian's hearing had not reached the level of the ninth-level ace Yan Xiaoyi, so he could deduce that his strength should be between the eighth and ninth-level. However, the specific level of the expert was not explicitly mentioned.