Elementary Statistics (12th Edition)

In the distribution, mean is $\mu$ and the standard deviation is $\frac{\sigma}{\sqrt n}=\frac{\sigma}{\sqrt {40}}$.
The number of students (40) is at least 30, therefore by the Central Limit Theorem the distribution of the mean ages is basically a normal distribution, where the mean is $\mu$ and the standard deviation is $\frac{\sigma}{\sqrt n}=\frac{\sigma}{\sqrt {40}}$.