The median age going up by .4 in a year is a substantial change. It mean that there is an increasing pace of older people on pensions and fewer of people of working age paying for the pensions. We've utilised immigration in the past to both slow that median age increase and to increase the ratio of those of working age paying taxes to maintain the pensions.
Ah, well, people want to live longer and not have children, or not too many, so economic affairs have to be arranged to support that. This means a mix of: working longer before retirement, smaller pensions, increased immigration from countries with younger populations, investment in and emigration to those countries and so on.
More or less as has been happening since the 60s ?
Of-course if we don't fix global warming, cooperatively manage the world economy and do just rely on herd immunity for pandemics - the problem will solve itself.