One factor that could explain why the Baby Boomer generation is so much larger than the Gen X, Gen Y, or Gen Z cohorts is the post-World War II baby boom. The increase in birth rates during this time period led to a significant increase in the population of Baby Boomers.
Gen Y and Gen Z will affect the age structure and growth rate of the United States by eventually becoming a larger portion of the population as the Baby Boomers age and eventually reach retirement age (65+). This will likely lead to a shift in the age structure as Gen Y and Gen Z cohorts become the dominant age groups in the population.