The mean absolute deviation of a data set is the average absolute difference between each data point and the mean.
First, find the mean of the data set.
There are 8 players in the chess club. Then, the average age of the players is:

Take the absolute value of the difference between the age of each participant and the average age of 37 years:

Next, find the mean of the absolute differences to find the mean absolute deviation:

Therefore, the mean absolute deviation of the data set is 12.25.