The myth of overpopulation is one propagated by the bourgeoisie in order for them to be able to kill certain percentage of the working class.

Basically, since labour is a commodity in the market, they need for it to have a surplus, so that thanks to supply and demand the price of labour goes down, and they can have cheap worker from where to choose. It's what Marx called the "army of the unemployed". At the same time, the bourgeoisie cannot allow for too many unemployed to exist, since otherwise there would be too much discontent and the working class would protest, or worse, and it's counterproductive for them. They need a delicate balance of employed middle class people and unemployed lumpenproletariat or low class people. The resources are there to feed everyone, we have worked out the scarcity issue, it's all artificial.

We indeed need young and productive people, but the reason why recent trends of natality have gone down is because living prices have gone up so drastically that people cannot afford to reproduce any more, and choose not to have children. There's no need for the elderly to go away, a source of income for young people could be precisely to take care of them, we just need to stop sustaining and working for a parasitic class that the only thing it does is live off the work of other people.