[Admin]

The query:

SELECT YEAR(NOW())-AVG(YEAR(birthday)) AS avgage FROM user;

You might want to round() it later, or you can do it directly in the query using MySQL's ROUND() function, for example:

SELECT [high]ROUND([/high]YEAR(NOW())-AVG(YEAR(birthday))[high],2)[/high] AS avgage FROM user;

will round it to 2 decimal points.