Question

6. A country that has a stable population is characterized by an age structure that is _____.

Answer 1

I suspect the answer is 'unchanging'...
http://en.wikipedia.org/wiki/Population_pyramid

Lotkai proved that the age distribution of any population
that is subject, for a sufficiently long time, to constant
fertility and mortality becomes fixed. He called
this end-product of constant demographic conditions a
"stable population", characterized both by an unchanging
age distribution and by a fixed annual rate of natural
increase