Puede hacerlo usando el siguiente SQL:
SELECT name,
(
SELECT datecreated
FROM tbl_comments
WHERE tbl_users.user_id = tbl_comments.user_id
ORDER BY datecreated LIMIT 1
)
FROM tbl_users
WHERE category = '1';
O usando:
SELECT tbl_users.name, MAX(datecreated) AS latestcomment
FROM tbl_users LEFT JOIN tbl_comments ON (tbl_users.user_id = tbl_comments.user_id)
GROUP BY tbl_users.name;