median function

In Column A I have the ages of a random number of people. I need to take the median of the top 25% of this list. First I sorted the list, and now i need to run Median(A1:AX) Where X is the cell that equals 25% of my total population. My problem is the population(and thus the number of rows used) is constantly changing. How can I put in a value for AX into this equation? Thanks in advance.
Karl