Excel formula to average selected cells based on certain criteria

Question

I'm working on Excel 2003.
I have a table.

Date - Column X - Column Y - Column Z - Age

When the date in the date column equals cell A1, I want to calculate the AVERAGE for the numbers in the age column.
There are many records; I tried using INDEX and MATCH but was unsuccessful. Only the first matching record is left for me.

Answer 1

Try:

=AVERAGE(IF(X:X=A1,Z:Z))  

With Ctrl+Shift+Enter.