Find results with greatest difference
The query below will return the maximum of the difference age from the guessed age:

SELECT MAX(DiffAge) AS MaxDifference
FROM (
    SELECT (`avg` - `age`) AS DiffAge 
    FROM `AgeTest`
) AS Result


Sample execution with given sample data:

CREATE TABLE `AgeTest` (`age` INT, `avg` numeric(18,2));

INSERT INTO `AgeTest` (`age`, `avg`) VALUES (18,34.45);
INSERT INTO `AgeTest` (`age`, `avg`) VALUES (32,33.23);
INSERT INTO `AgeTest` (`age`, `avg`) VALUES (45,28.21);

SELECT MAX(DiffAge) AS MaxDifference
FROM (
    SELECT (`avg` - `age`) AS DiffAge 
    FROM `AgeTest`
) AS Result


Working Demo: http://rextester.com/BSEEG25743