order by a "select as" determined by case statement

I'm not a db guru by any means so please forgive me if this has an easy
solution.

Scenario:

Want to sort by an alias for SELECT AGE(). Problem is the timestamps for
the SELECT AGE will be determined by the value of another column, thus
the use of CASE.

What I thought would work.

SELECT tblticket.ticketid,tblstatus.statusname,
CASE WHEN tblstatus.statusname = 'Closed'
THEN (SELECT AGE(tblticket.ticketclosed,tblticket.ticketcreated) as
ticketage)
ELSE (SELECT AGE(CURRENT_TIMESTAMP,tblticket.ticketcreated) as
ticketage) 
END
FROM tblticket CROSS JOIN tblstatus
WHERE (tblticket.reasonid = 2) AND blah, blah, blah
ORDER BY ticketage ASC

CASE apparently completely ignores the ticketage alias. Anyone know how
to get around this?

Thank You.