Limit is the number of results returned and you must SET the limit on your query before run it.
See this:
Try this:
--[params]
--string :interval = 1 week
--int :limit
SELECT tu.topic_id, t.title, t.created_at as posted
FROM topics t, topic_users tu
WHERE t.id = tu.topic_id
AND age(t.created_at) < interval :interval
AND t.archetype != 'private_message'
GROUP BY tu.topic_id, t.title, t.created_at
ORDER BY t.created_at DESC
LIMIT :limit