Without clear evidence, I wouldn't attribute this to people becoming healthier. The increasing availability of alternatives (i.e. legal marijuana) and demographic changes seem like more probable factors, as the article suggests.
A more controversial claim might be that decreasing levels of socialization, or shifting leisure preferences, among the youth also factor in. There have been plenty of studies in recent years that explore the seemingly-relevant decrease in sexual activity within that age group, for example (https://www.google.com/search?client=firefox-b-1-d&q=young+p...).