Five years old. Professor Liu Yu from Tsinghua University wrote a letter to her daughter on Children's Day on June 1, 2022, which included "Happy Fifth Children's Day to her daughter", and the letter showed that her daughter was 5 years old. Liu Yu, female, from Poyang, Jiangxi,1975,65438+February. Graduated from the Department of Political Science, Columbia University, USA, with a postgraduate degree and a doctorate in political science.