The mastoid develops from a narrow outpouching of the posterior epitympanum named the aditus ad antrum. Pneumatization takes place shortly after birth, after the middle ear becomes aerated. This process is complete by the time an individual is aged 10 years. Mastoid air cells are created by the invasion of epithelial lined sacs between spicules of new bone and by the degeneration and redifferentiation of existing bone marrow spaces. Other areas of the temporal bone, including the petrous apex and zygomatic root, pneumatize similarly. The antrum, similar to the mastoid air cells, is lined with respiratory epithelium that swells in the presence of infection.
Blockage of the antrum by inflamed mucosa entraps infection in the air cells by inhibiting drainage and by precluding re-aeration from the middle-ear side. The mastoid is surrounded by the posterior cranial fossa, the middle cranial fossa, the canal of the facial nerve, the sigmoid and lateral sinuses, and the petrous tip of the temporal bone. Mastoiditis can erode through the antrum and extend to any of the above contiguous sites, causing clinically significant morbidity and life-threatening disease.
Ok, I know that’s a bunch of crazy doctor-speak if you haven’t specifically studied the ins and outs of the anatomy of the skull, but the development of the ear takes a long time. The mastoid isn’t fully formed at birth, far from it.
Acute mastoiditis can easily take hold during untreated middle ear infections, when the mastoid is being formed and the immune system isn’t yet up-to-speed (almost always before 2 years of age). Chronic mastoiditis is more complicated, but is still generally influenced by the fact that the mastoid is still forming.
Though antibiotics can almost always clear up ear infections before mastoiditis sets in these days, before their advent, mastoiditis was actually a very significant cause of childhood mortality. Since the mastoid process is so close to other critical parts of the skull (namely just a thin bone away from the brain, and right next to cranial nerve VII), complications were not uncommon, and were not easy to survive.
This x-ray shows one of the more common complications that killed children.
From Monographic Medicine: The clinical diagnosis of internal diseases. L.F. Barker, 1917.