A university president believes that over the past few years, the mean age of students attending his university has decreased. To test this hypothesis, a study is conducted in which the age of 150 students who have been randomly sampled from the student body is measured. The mean age is 21.5 years. A complete census taken at the university a few years prior to the study showed a mean age of 23.5 years, with a standard deviation of 8.5. Test using a 1% level of significance.

What is the null hypothesis?