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.
https://covid19.sccgov.org/dashboard-case-rates-vaccination-...
Kill pathogens more quickly, you'll be infectious for less time, and whoever do you infect will have a lower viral load.
Source: https://www.cdc.gov/coronavirus/2019-ncov/science/science-br...