I had the chance to sign up for a 401(k) plan when I was still in high school. I landed a job at a bank my senior year of high school and worked on work release to finish out my senior year. I simply didn't want to go to school anymore. I was lucky because I was able to transfer down to where I went to college and I had a much better paying job then most of my college friends. But I didn't invest in retirement. Looking back I really wish that I had. The sooner you start, the better off you are.
I started paying attention to retirement contributions when I landed my first job after I graduated. The sooner you start the better off you will be, and you will not need to spend as much trying to catch up when you are in your fifties and sixties. Once you start a family and take on additional financial responsibilities like owning a home, it can be a lot harder to find the extra money in your budget to put towards retirement. If you start when you first get a job, you will not miss the money and you can set up a steady pattern of contributions to your retirement accounts.