Making Courtney 17 disagrees with previous canon - as noted below, she's explicitly stated as 16 at the time she was involved with Billy (and I'd have to dig out my copies of Stars and STRIPE to check, but ISTR that she was noted as 16 there, too), she was already looking into colleges as of Infinite Crisis, making her 17 or extra-studious and about to turn 17, before the jump ahead.
Making JSofA 26 (which showed her last birthday) her 18th or 19th birthday. 20th would also be valid, given the vagaries of the post-gap timeline, if she weren't shown getting off a schoolbus at the beginning of the issue.