Sometimes, mastoiditis can be misdiagnosed. It requires the use of a bone scan, not MRI, to be seen. Doctors look for the appearance of liquid in the mastoid bone which is assumed to be infection. Rarely, that liquid may not be an infection, but the extension of liquid in the inner ear.
I was misdiagnosed with mastoiditis but a second scan confirmed that it isn't.
From what my doctor says, mastoiditis is very rare in adults. Mainly because ear infections or other related infections that lead to mastoiditis cause symptoms that will cause people to see a doctor. There will be pain, differences in hearing, headaches, etc. Not having or noticing these symptoms until the infection gets bad enough to go to the mastoid bone is unlikely.
It's harder for children to take notice of these symptoms and express them and they also experience more ear infections like the article said. So that's why they're more prone.