They won't be penniless. What they will be is the single largest voting block of all-time. It's not a great time to be 25-35 years old and thinking about how this plays out.
That assumes there will be a democratic system. If the economy collapses revolution and warlords will control.
I'm surprised that 40% was the number given. I bet it is more like 80%. As far as the planet being overpopulated, I would definitely say no. I have done some travelling around and their are huge expanses of empty land out there just ripe for settlement. I believe that the Earth could support about 200 billion people comfortably. Maybe the area you live in is overpopulated but I wouldn't say the Earth as a whole is.