What happens when people get older? Can you still identify it's the same iris when the iris was scanned 15 years ago? I don't get what they are trying to solve

What happens when people get older? Can you still identify it's the same iris when the iris was scanned 15 years ago? I don't get what they are trying to solve. Just fair distribution of coins? Once everyone has an equal number of WorldCoins, then what is the plan?