Kids can also be carriers, and more spread among a partially resistant population means more mutation and more variants. Vaccines are in the process of being approved for all ages.
Kill pathogens more quickly, you'll be infectious for less time, and whoever do you infect will have a lower viral load.
https://covid19.sccgov.org/dashboard-case-rates-vaccination-...
Source: https://www.cdc.gov/coronavirus/2019-ncov/science/science-br...