From what I understand, any type of training between 2 mile pace and half marathon pace is good for improving VO2 Max. A good example of a VO2 Max session would be something like 5 x 1000 at 5k pace, with 1 or 2 minute recoveries. I have no idea what the average VO2 Max is for teenagers, and I don't really see much value to it. There are so many other factors that influence your ability as a runner besides VO2 Max. You could blow someone else out of the water even if their VO2 Max is higher than yours if you have a better lactate threshold, better running economy, etc.