I know the title of this thread referred to the age at which you start talking about college to your child, but I think what Cati was talking about with her child was working hard and doing your best. As she stated in several posts - it was not about an absolute grade but about an approach to doing the work that week.
Instilling good work habits in children is one of the most important things that a parent can do. Lots of you have posted about how not everyone has to go to college, but we all have to do something in the grand scheme of things.
The plumber, the mechanic, the retail clerk, the SAHM, etc who doesn't necessarily have to a degree to be successful still needs to know what good work habits are. You don't want to hire a plumber who just kind of patches things up with the least amount of effort because fixing the real problem would take a lot of work. A SAHM needs to be very self disciplined and has to work VERY hard to do her job well.
You can't just sit down and dictate that your child must attend Stanford or Princeton. There's a certain amount of self-motivation that has to be present. But I don't see any harm in helping a child understand that he or she should try to do their best and that there are life consequences from the decisions that they will be making.