I have 1987 Juniper Bonsai that has great sentimental meaning, I also had
another much young Juniper Bonsai - both of which seem to be suffering from
some sort of definciency - the leaves(needles?) on the younger started to die
and eventually the tree died. My second, much older tree has now started to
turn brown too and I'm not sure why,
2) The dying Juniper has been in the same location for over 5 years and was
fine until very recently

There's your answer. I bet you are keeping it indoors. Junipers are outdoor
trees and need a cold dormant period every winter. It is probably dead. Scratch
the bark. If there is green underneath you may save it. Put it in the coolest
place you can find but not below freezing. Next spring put it outdoors & leave
it out, until around Thanksgiving when you will put it in a sheltered but cold
spot for the winter, like a garage.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"A tree never hits an automobile except in self defense." - Woody Allen