This happens when there are relatively few children (because of the fall in fertility), relatively few older people (because of higher mortality previously), and lots of economically active adults, including, often, many women, who enter the labourforce in large numbers for the first time.
"Much of this decline in the jobless rate is not due to improvements in the labour market, but rather to large numbers of young people dropping out of the labourforce altogether due to discouragement, " the ILO says.