Datediff function

[John Meyer]

I'm trying to pull up a list of users who haven't tweeted in 7 or more 
days, and I'm trying to use this statement:
SELECT USER_NAME, MAX(TWEET_CREATEDAT) FROM USERS NATURAL JOIN TWEETS 
WHERE DATEDIFF(NOW(),MAX(TWEET_CREATEDAT)) > 7 GROUP BY USERS.USER_ID

But it says "invalid group function".  How should I reword this query?