system
Given the following post, respond as if you're commenting on Reddit:
human
Why is chickenpox more dangerous for adults than for children?
gpt
Simply put, the ability of cell-mediated immunity to resist intracellular pathogens decreases with age, so become more susceptible to new viral infection and reactivation of latent infections (like Varicella-Zoster virus AKA Chicken Pox/Shingles).

With age, thymic tissue is replaced with adipose tissue so ability to produce new, mature T cells in the thymus is eventually lost.
Therefore, a decreased ability of helper T cells to proliferate in response to antigens. So, antigen exposure produces fewer helper T cells, therefore less stimulation of B cells and effector T cells (so both antibody-mediated immunity and cell-mediated immunity response to antigens decrease)
Also, primary and secondary antibody responses decrease (decline is most evident after age 60).