The following will give you a count of patients with activity in calendary year 2006 who were between 13 and 19 on the last day of 2006.

SELECT
COUNT(distinct a.ptnum) AS "numpts"
FROM
journal a
JOIN patients b on a.ptnum = b.ptnum
WHERE
trantype = 's'
AND amount > 0
AND trandate BETWEEN '2006-01-01' AND '2006-12-31'
AND AgeInYears(dob,'2006-12-31') BETWEEN 13 AND 19