This doesn't pass the smell test. Human productivity hasn't been a function of food calories and muscle mass (i.e. healthy 15-35yos) for at least a century.
While America might have a marginally younger population than China or Germany, it is not even close to competitive in terms of actual productivity. Among the younger generations there is a lot of confusion and feeling lost. Anecdotally, almost everyone from my high school graduating class -- young people! -- are floating aimlessly through university, dead-end service jobs, or unemployment. They are not "productive" in any real sense of the word. Unless there is a revolution in public policy and culture within this decade, America will face a genuine economic crisis long before Germany or China. Many parts of America (Rust Belt) have been feeling it for over a decade.
Looking solely at the two-dimensional demographic chart is a fatal error in the 21st century.