Why mandate young borrowers to contribute to their retirement accounts?

Many countries, in an effort to address the problem that too many retirees have too little saved up, impose mandatory contributions into retirement accounts, that too, in an age-independent manner. This is puzzling because such funded pension schemes effectively mandate the young, who wish to borrow, to save for retirement. Further, if agents are present-biased, they disagree with the intent of such schemes and attempt to undo them by reducing their own saving or even borrowing against retirement wealth. We establish a welfare case for mandating the middle-aged and the young to contribute to their retirement accounts, even with age-independent contribution rates. We find, somewhat counter-intuitively, that pitted against laissez faire, mandatory pensions succeed by incentivizing the young to borrow more and the middle-aged to save nothing on their own, in effect, rendering the latter's present-biasedness inconsequential.