The difference isn't all that noticeable, to be honest, or at least I've never found it so. If you're using older hardware, you're going to get an older "experience" anyway. The most user-visible kernel improvements tend to be improvements in hardware support, which is irrelevant if your hardware is already fully supported. However, I don't do anything fancy with my machines—no full-disc encryption or the like. I usually don't even need an initram to boot the system. So maybe you would notice something if your machines were more complicated.
(Note that the laptop I mentioned above started out with, um, a 3.x kernel? It gets a new one every year or so. The only kernel changes affecting it that were significant enough to draw my attention since 2008 were a fix in the support for the Broadcom wireless card it carries, and some changes to how hibernation works, which didn't matter in the end because I basically never did try all that hard to get hibernation working on that machine.)
You're unlikely to have issues unless an entire architecture loses support from your distro, and if you're running x86_64, that isn't going to happen for a long, long time. I've never been in a position where I couldn't compile a new workable kernel for an existing system out of Gentoo's repositories. The only time I've ever needed to put an upgrade aside for a few months involved a machine's video card losing driver support from nvidia—I needed a few spare hours to make sure there were no issues while over to nouveau before I could install a new kernel.
Note that you can run an up-to-date userland on an older kernel, too, provided you make sensible software choices. Changes to the kernel are not supposed to break userspace—that's meant to keep older software running on newer kernels, but it also works the other way around quite a bit of the time.