SELECT * FROM table WHERE (NOW() - datefield) >= INTERVAL 5 DAY;
SELECT * FROM table WHERE datefield + INTERVAL 5 DAY >= NOW()
The best way to do this is to calculate the actual date value that marks '5 days ago'. ie: NOW() - INTERVAL 5 DAY:
SELECT * FROM table WHERE datefield >= NOW() - INTERVAL 5 DAY;
This way the database will see that NOW()-INTERVAL 5 DAY is a constant within the transaction, and calculate it only once. Comparing a date field against a constant can use an index so this is the fastest way of doing this.