The RHEL (and CentOS) 5.x series use GCC 4.1 and do not provide newer versions because they are stable, "enterprise" distributions so you can't update to a newer version using yum
.
(If you want a newer system compiler then don't use such an old "Enterprise" distribution, either use CentOS 6 or a non-enterprise distro such as Fedora).
It's easy to install a newer GCC elsewhere on the system and then use that instead. Just download the gcc-4.9.0.tar.gz file from a GNU mirror and read Installing GCC for simple instructions on building it. It is a bad idea to overwrite the existing system compiler, you should install to /usr/local
or /opt
or another directory. You can then alter your $PATH
so the newer GCC will be used. You should also read about setting dynamic library paths in the libstdc++ FAQ.
Another alternative is to install the Developer Toolset, which is an optional add-on providing a recent version of GCC and other dev tools, see http://linux.web.cern.ch/linux/devtoolset/ for a re-packaged version for Scientific Linux that should be compatible with CentOS.