An aging and slow growing population can have very negative effects on a country's economy. Japan is a great example of this type of effect. As the population ages, elderly citizens require more care, which is expensive. With a slow growing population that means there are fewer people to take care of an aging population.
This can make it harder for some able-bodied people to work if they are needed at home to take care of an aging parent or grandparent. This also negatively impacts the economy. Countries that pay for elder care from government programs will see the costs of these programs increase while contributions to the program from younger people's paychecks decrease.