Final answer:
The thymus becomes much smaller during adulthood in humans. It undergoes a process called thymic involution, gradually decreasing in size from birth. This process continues throughout adulthood at a slower rate.
Step-by-step explanation:
The thymus becomes much smaller during adulthood in humans. It is most prominent in infants and undergoes a process called thymic involution, where it gradually decreases in size starting from birth. This process continues throughout adulthood, with the thymus shrinking at a rate of about three percent tissue loss per year until around 35-45 years of age, after which the rate declines to about one percent loss per year for the rest of one's life.