This shouldn't be a problem as long as they don't panic. It will have an increasing social cost to support the elderly, but at the same time, in theory they worked all their life to save up and contribute to (state) pensions to afford this. In theory, in practice (as a layman) it seems the younger generation pays for the older generations.
Anyway, this will happen all over the world over time; birth rate is dropping in Europe and N-America because of the current economic climate, that is, people can no longer afford to have children, because they have to work tons of hours and multiple jobs just to keep afloat. The boomers were able to buy a big house and cars in the suburbs in their early twenties on a blue collar job. Not going to lie, I'm jealous of that.
Meanwhile, I was a single white collar worker who had to live in substandard 'student' housing for close to a decade before finally my wage and the housing market aligned enough for me to be able to buy a modest house. Not complaining about the house or anything, but I already feel very privileged and since then the housing prices have gone up by another 10-20%, making owning a house and therefore the stability to start a family unreachable for a lot of people.
What I'm trying to say is, to slow down or reverse birth rate decline in 'the west', we need financial stability and security. We need to be able to afford a decent house on a single income, and to get rid of the stress of living in debt and from paycheck to paycheck.