When do the tonsils usually shrink?

By preteen/teenager: At birth, your tonsils and adenoids (t&a) are small and your immune system is immature. Your t&a are immunological structures and make antibodies to pathogens, like bacteria and viruses; so as you grow, they grow and enlarge, doing their job. Our immune system is relatively mature at 5 to 7 years of age, when the t&a peak in size. They then usually shrink to become small again by preteen/teenager.