Imagine a desert island of two people. They have a system of savings. You can bank away a voucher that makes the other person gather coconuts for you, and do other necessary work.
One of the two people could conceivably work harder in their youth and then work less later using the vouchers. But:
1. They can't both do it 2. If the other person loses capacity to work hard in their later years, your savings voucher loses value.
How can they both save? To a limited extent, they can store resources like extra coconuts, build infrastructure and tools while young, etc. But this only goes so far: food rots, infrastructure depreciates.
On a global scale the world is no different. A savings shortfall means that global youth productivity won't be high enough to support global elderly retirement. We will need to either:
1. Get youth more productive 2. Make more youth 3. Take a greater percent of youth's income 4. Have old people live on less or not retire so early
Notice that none of these are about saving now? That's because moving 1's and 0's in a ledger isn't a provisioning of future goods. You need actual future goods production.