Reasons for this is cause an app I use supports only older kernel versions.And I need newer Ubuntu versions cause they're supported(still have repos)and support for gnome shell.I currently use Ubuntu 10.04.
Download this(I think it's the oldest version supported). https://www.kernel.org/pub/linux/kernel/v2.6/longterm/v2.6.34/linux-126.96.36.199.tar.xz
Hold down the shift key while booting and it will display the Grub menu. You can now select an older kernel version.
For selecting an older kernel as the default kernel see this post.
Be sure to don't update the kernel no more.
We have used older kernels on newer userspace in some circumstances. As long as the delta is not too great we might expect it to work fairly well. It would of course not be supported in this combination. If the application you have works with 10.04 and is likely to work with 12.04 then you are likely best off waiting until 12.04 support is available.