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.