Since I've become an adult with habitual patterns, I've been able to tangibly gauge the effects of caffeine on my mind and I'm not entirely sure that young people, what with all the other natural chemicals raging through their body, are able to do the same.
I'm a person that's tangibly sensitive to the effects of caffeine. For me, a single morning cup of coffee can immediately waken me to the point where I'd be if I naturally allowed myself that one hour transition from waking to working. However, a second cup of coffee within a 5 hour period will cause my mind to race faster than I can effectively communicate and I have 'the jitters'. Also, a large cup of Starbucks will literally make it impossible for me to work as I can't focus on my screen or conversations. And if I have a coffee after 3:00 PM? I can forget about sleeping before 2:00 AM.
What works best for me, what allows me to reach an effective plateau, is one cup of coffee in the morning and a cup of green tea (or yerba) in the afternoon - I'm always sharp, on the ball and I'm less distracted by whatever causes ADHD.