Update Computed field with hook_cron
From your question, by hook_cron() this can be done. You can use a watchdog() function to make sure that the cron was run. The computed field has a table entry in the database, so you just need to find out that one and check each user's birthday from a DOB field stored in another table based on UID. Then if the user has updated the DOB then check for their ages and insert/update the value in the "field_age" computed field. All of the operation can take place in hook_cron() which will be invoked by cron.php each time the page is fired in crontab.