19.10.2010
This can be done calculating the difference in years, and then, taking into account the day of birth function of today: if in the future, we should -1 from age, because that person isn't there yet ;)
SELECT ((DATE_FORMAT(NOW(),'%Y') - DATE_FORMAT(date_of_birth_field,'%Y')) - (DATE_FORMAT(NOW(),'00-%m-%d') < DATE_FORMAT(date_of_birth_field,'00-%m-%d'))) AS age FROM some_table WHERE 1
PostgreSQL has a nice function called age.
SELECT EXTRACT(year FROM AGE(NOW(), date_of_birth_field)) FROM some_table WHERE 1