TSQL - Average of all values in a column that are not zero

Question

I\'m in the process of writing a report and am looking to get the average value of an age column. The problem is that not all rows have an age.

If the values for the

Answer 1

, ( SELECT AVG(a) FROM

        (
        SELECT NULLIF([Column1], 0)
            UNION ALL
        SELECT  NULLIF([Column2], 0)
            UNION ALL
        SELECT  NULLIF([Column3], 0)
            UNION ALL
        SELECT  NULLIF([Column4], 0)
        ) T (a)
    ) AS [4 Column Average]