I have 12.04(for ia32-libs) and 15.04 beside win8 and XP :)
When I update the Ubuntu, for example 15.04, the kernel changes form 3.18.x to 3.18.y, while Ubuntu 15.10 uses kernel 4.2.x. Ubuntu 12.04.5 support is till 2019 and they are available(thanks to it's developers), but the kernel updates are minor for example: from 3.19.x-20 to 3.19.x-22.
There is lots of bugs which has been fixed in the newer kernels.
Why the oldest versions can't migrate to newer kernel?
I need 12.04 for it's great compatibility with 32bit apps, but I want the latest stable kernel. I know I can build the kernels for my Ubuntu, but the officials are better than mine.