There are many reasons why teenagers or really anyone at any age might be considered lazy. Some different phases and behaviors occur during various stages of our lives. Teenagers go through a time when they will show a lack of interest in schoolwork, chores, and community life.
Adults have already been through this and know the “drill” regarding juggling work and life’s pressures. Children have to learn about time management, organizational skills, and just dealing with school pressures, home life, peer acceptance, and everything that goes with their present world.
If you want to get into the scientific aspect of it, you have to consider hormonal changes that cause a shift in their internal clock, which tells them to sleep later in the day and wake later in the morning. The brain is not fully developed until your mid-twenties, and some school systems begin at the crack of dawn, so you have a teenager that could fall asleep in class, projecting the picture of laziness.