Is 160 pounds an average weight for an 18-year-old?

It depends on the height and body composition of the 18-year-old. Weight is related to height and body composition rather than age. For in-depth information about weight in relation to height, and to see charts showing the distribution of weights and heights of children and teenagers at various stages, please see the page link, further down this page, listed under Related Questions. Alternatively, please feel free to ask the question again and include more detail.