Given sufficient energy, enthusiasm and patience it's never too late to change careers. If you can accept the lower wages, lower levels of responsibility and somewhat reduced expectations of finding entry-level positions in IT right now, you will eventually be able to make the change you seek.
Your progression through Cisco infrastructure admin to design and security sounds pretty good, except that I'd urge you to also obtain a CCNP (Cisco Certified Network Professional) cert; by itself, the CCNA isn't worth much except as a pre-requisite for other Cisco credentials. You can do the CCDP (Cisco Certified Design Professional) by all means, and then pursue the Cisco security specialist certification.
I'd urge you to also try to find some work as a network or infrastructure technician, help desk person or administrator ASAP because experience will count as much (or more) than whatever certifications you earn. If you can get to work in the field in the next 12-18 months and finish your Cisco Security Specialist exams in 24-30 months, you should be well positioned to find work in the security field. To get yourself started down the security path, you might also want to consider the CompTIA Security+ or the Cisco CCSP (Cisco Certified Security Professional) as lead-ins to the Cisco Security Specialist credential.
This was first published in February 2003