Human growth hormone from the pituitary gland naturally peaks by around age 18, you coast on that high until about 25, and then it starts to noticeably decline past 30. That said, cells are constantly dividing and being renewed until you are dead, and HGH can be stimulated by intense exercise, trauma, and sleep(!) at any age. Most physiological decline associated with aging is largely the result of sedentary lifestyle, not a biological imperative. So you biology teacher is wrong, and there are scientific experiments that prove it if you want to do the research yourself.