[CentOS] Is every CentOS release supported for 7 years?

Thanks for the quick replies. I understand now that CentOS  5 and all
5.? versions are supported until 2014. How does this work with
security updates? Does each point release gets itś own security
updates? In other words is it possible to install
CentOS 5.5 on a server and only apply security updates for 7 years? Or
is it required to upgrade to each point release in order to continue
receiving security updates?