What might be the economic effects for a society in which the average age is very young or very old?

The effects on society of a young workforce will be very
different from those of an old workforce.  In general, the younger workforce is much
better for a modern country.

The reason for this is that
the people who are of working age must support all the other people, both younger and
older, in a society.  Today in the US (though not as much as in places like Japan) we
see the difficulties that arise when the number of younger people falls relative to that
of older people.

Older people, on average, require more
health care.  This holds true both for older workers and for retired people.  As people
age, they retire and need pensions.  All of these things must be paid for by those who
are working.

A society with a young workforce is a society
on the rise.  It is one in which there are many workers to support the other people.  It
is one that is not likely to be heavily burdened by health care costs.  A society with
an older workforce is one that has more problems.  It has to, like our society today,
find ways to pay for the medical care (Medicare) and pensions (Social Security) of the
older people even as the number of workers to pay for these things
dwindles.