This is a personal preference. Some start young, some never choose this option. Much debate exists, but you need to decide if it is right for you and your child.
There is no set age on this, it's completely up to the parent(s). Personally, we don't let your six month old son (nor did we with our three year old daughter). Crying is one of the few signs a baby can show they need something, and I wouldn't ignore this.