Excluding the unborn, the dead and the distant...
In common usage, contemporaries are
The question of whether or not lifetimes overlap is more delicate than might
be supposed. If two objects are widely separated in space
what does it mean to say their lifetimes overlap. That statement presupposes
that we can draw a spacetime diagram with a time axis that applies both over
here and over there. This is a theoretical impossibility unless we and the
two objects are at rest relative to one another, as indicated by the
of simultaneity article in Wikipedia.
As an approximation, if we assume that that the speeds involved among the
observer and the two objects are much less than the speed of light, we
might be able to disregard any slight aberation caused by the relativity
of simultaneity. Even in that event, declaring our two objects contemporaries
needs some more thought.
Clearly if one object dies before the other is born they cannot
interact. But suppose that the two objects
were one light-year apart in space. If their lifetimes do
not overlap, in the reference frame in which they are both approximately at rest,
by at least by at least two years, there is also no way in which the objects can
interact. The is simply not enough time for a signal to get from one to the other
and back. I think that if no interaction is physically possible we might
reasonably conclude that the objects are not contemporaries.