Like a few of these people said, it depends on the level of maturity you have at the age you start. It also depends on how developed your social, educational, mental skills, and intellectual level is/are. Personally I know some people who are really young, but are already at the college level. However we must all remember that immaturity is everywhere these days, so we can't base our answers solely on that. I think a good age to start college would be from 13 to 16 years old.
Right after high school but after you get things together and know what you want to do. If your not planning on having a kid i would say 18. If you are planning to move somewhere else and go on a trip i would wait a few years maybe 21.
When you're ready. My cousin went to college at 16. I went at 18, after high school. A friend went at 22, after working a few years because she just wasn't prepared for college yet.
The plus of going earlier is that there are more scholarship opportunities for recent high school grads. The cons are that you may not feel prepared, or not be mature enough to take it seriously and drop out. You also don't have any time to save up for college while in the work force.
The plus of going later is that you are more prepared, know exactly what you want and have had time and experience in the work force. A lot of people who wait are actually able to get their degrees faster than younger students following the same path. The cons are that you won't have as much financial aid available (except for a few specifically geared towards older college students), and you will be a little older than most people so may feel a little left out.