name,   age,gender
"John", 33,m
"Sam",  33,m
"Julie",33,f
"Jimbo",, m

SELECT AVG(age) FROM [peterprivatedata.testpeople]

SELECT sum(if(is_explicitly_defined(numeric_field), numeric_field, 0)) /
       sum(if(is_explicitly_defined(numeric_field), 1, 0))
    AS my_avg FROM your_table

SELECT sum(if(numeric_field_is_null, 0, numeric_field)) /
       sum(if(numeric_field_is_null, 0, 1))
    AS my_avg FROM your_table