CentOS 5 reaching EOL (end-of-life) on 31st March 2017, after that software developer will stop supporting it and deprecate its use.
The CentOS released in April 2007 received full updates till 2012 after that only maintenance and security updates were received.
With this, cPanel would also no longer support it. Continuing with non-supportive and un-maintained OS can lead to security vulnerabilities and won’t be fixed.