In most systems your pension taxes are not stashed away until you retire and then handed to you. Instead your taxes are used to pay the pensions of people currently retired, with the understanding that the next generation will pay for your pension. If you stop working too early and you stop paying taxes, the system breaks down.

That said, I really don't think that this is a real problem. The real problem is that baby boomers are now retiring in droves, turning from the major contributors of the pension system to the biggest drain, and with a population shrinkage, it's uncertain how we'll be able to keep funding the system.