Riding at your max heart rate can in fact be dangerous and it is why stress tests are done in a medical office. I finished a race, got off the bike and had a stroke. I peaked about 5 beats under my max heart rate. Luckily the stroke was in a place where the result was not extreme but it could have also resulted in much higher drama. At age 15 there is little danger but as one gets older our blood vessels are naturally less flexible and extreme effort can be an issue. Now I limit my heart rate to high but short of a totally max effort. That was disappointing to be forced to do because I enjoyed pushing myself to a maximal effort.