Why is it important to start saving for retirement when you're young?

So, you will have the money when you retire. If it isn't there you can't retire and have to keep working. The older you get the harder it is to work and to live. Medical expenses go up for you and you still need to eat, pay for housing, have a car, and everything you needed when younger that does not change.

Money invested at the beginning of the earning cycle will earn much more interest. A few hundred dollars invested immediately upon entering the work force is going to create much more of a return than thousands put away decades later.