There were 9 people at a birthday party. the median age was 3 years and the mean age was 19 years. what could account for such a wide difference between these two measures?

The average is the total age divided by the number whereas the median is the middle number. The median is not directly affected by the other numbers, only the placement of them.  The mean would include any outliers unless excluded from the data - in this case they would not.
The median is better with skewed information - as no outliers will affect it.

What’s the question here? what have you been learning about?