A child or a young person — roughly anyone from a small child up to the late teens, though nobody draws a firm line. The important thing is not the age but WHO says it. "Youngster" is almost always spoken by an older person looking at a younger one, often warmly, sometimes with a touch of "in my day…". A grandfather says "the youngsters these days"; a teenager never calls a friend a youngster. It sounds slightly old-fashioned and a bit newspapery — you see it a lot in local news and sport ("the 17-year-old youngster made his debut") because it is a handy way to avoid repeating "young player". It can carry a faint suggestion of inexperience, which is why calling a young adult a youngster can sound gently condescending. It also gets used for young animals, especially in nature writing: "the mother bird feeds her youngsters". Safer everyday choices are "kid", "child", or "young person", depending on how formal you want to be.