Running older kernels: Is it safe?

Kernel 2.6.22 is inside Ubuntu Gutsy Gibbon. Gutsy will be supported for another eight months. After this security updates will stop and with this the support for kernel 2.6.22.

So the fact that I am running Hardy does not change the security of running 2.6.22, and therefore I can boot 2.6.22 safely as long as Gutsy is supported?

How did you install kernel 2.6.22? You've got to keep in mind that only software which you installed via the repositories gets automated updates. So if you installed kernel by compiliing of by single .deb files you won't get automated updates. And therefor no security updates.

I didn't do anything special. I installed 7.04, upgraded to 7.10 and then to 8.04. All I know is that if I hit 'esc' at boot I am offered a choice of kernels. If I don't hit 'esc', 2.6.24-19 is booted.

I would like to boot 2.6.22-##.

It is safe to run older kernels, minus any bug fixes and security holes that have since been patched. The best advice is to stick with what works best for you. When that kernel is no longer available in the repositories, it will stop being supported, but by then you should probably have moved on. Since you are using Hardy, you will ultimately want to use the kernels that came with hardy (2.6.24-19 currently), and these will be supported for the 3 year desktop lifetime of Hardy Heron.