Version is widely reported as one of the last stable versions for Lollipop. Find older versions on APKMirror.
Only if you have no other choice. For YouTube, sideload v17.09.37 now, but plan to switch to NewPipe soon.
: This is a popular open-source, lightweight YouTube client. Older "Legacy" versions of NewPipe often work well on Android 5.1.1 and offer features like background play and no ads.