I got certified many moons ago (1969), at the age of 16. Back then, teenagers just did not go through the rigorous training that was done by NAUI to become certified. The classes and in water training were done, in my case, by instructors who were former Navy SEALS, and were quite hard core. The first couple of classes dealt only with swimming skills, including treading water without using your hands while passing a 15 pound weight belt to each other. Needless to say, you were ready to move on to snorkeling and then SCUBA. One of the hardest parts for me was jumping into the pool holding all my equipment, putting it all on and clearing the snorkel and then regulator in one breath. Back then, while performing this checkoff with SCUBA, the instructors would harass you by tugging at your fins, pulling off your mask, and even turning your air off. This was to prepare you for the emergency that could happen. I was a very nerve wracking part of the course. I did pass though and enjoyed many years of diving.