All centos 5.x kernels are based on 2.6.18, there aren't kernels based on 3.x not 4.x codebases for centos 5.
Another important factor you should consider is that centos 5 reached EOL (End Of Life) about 1 year ago:
Mar 31, 2017 EOL
You can can confirm this on the CentOS wiki: https://wiki.centos.org/HowTos/EOL
This means that there are nor will be any more official updates of any kind for CentOS 5. It is past time for you to update to a newer system. Your system is completely exposed to every known vulnerability that came out in the last year.
Expanding on this:
Centos 6 has already reached the end of full-updates support on May 10th, 2017, but it will keep receiving security updates until November 30th, 2020 (EOL)
Centos 7 will have full updates till Q4 2020 and will reach EOL by June 30th, 2024