No, a 15 year old child is a minor, not an adult or even a young adult.
A young adult is an individual who's age is 18, 19 (and still teen agers, as well), 20, 21, 22, 23, 24, and 25. You see, the human brain is not fully developed until the age of 25. Therefore those 18 to 25 are young adults. Also, teenage adults (18 and 19) as well as young adults, are still growing teeth (molars).