What is the earliest age for a person to be able to visibly build up their six-pack abs?

Pre teens: Not that it's important, but there is at least one case of a 10 yo ripped body builder. I would not aspire to such a build. It generally requires you drop your total body fat to the 5-8% level. Most normal folks run about 20%. This is not just a workout thing it requires dietary restrictions that could prevent a healthy transition through the teen years, final height and muscle mass.