In the United States, what is the average age (or age range) by which
a child can expect to have outlived their parents? In other words, on
average, by what age (or age range) have most US citizens experienced the death of
both parents?

This looks easy enough ...

If the average age of a parent is 24 and the average age of death is
64 then the average age of a child on the death of a parent would be
40.

(Give or take a few hours)

The average age of death in the US is 64? Isn't it closer to 77? (See
National Center for Health Statistics.

It's not that easy probono.

Firstly, perhaps the average age of death is different to the average
age of death for those who have kids.

Secondly, even if my first point is negligible, then your analysis
would only work for Amoeba/Some plants/Other Asexually reproducing
species which only have one parent.  The original poster wants to know
average age at which both parents have died.  Therefore we must know
something about the distribution of death ages, because it is
generally rare that both parents die at once.

Thirdly, the offset between average fathering age and average
mothering age also matters.

My point is, no easy calculation will suffice to answer this question.
 Instead I think some real statistics directly answering this question
are required.

If we assume for the sake of the argument that the average age of
death is the same for those who have kids as for those who don't , AND
we assume that the age difference between the two parents is within 5
years of one another --- does that make it any easier to answer the
orginal question?

I tend to agree with the forumla offered above as the best possible
means of reaching an answer in lieu of finding hard statistics, though
one could probably find more reliable data to support the various ages
of life expectancy, etc.

Regards;
tutuzdad-ga