Senior Citi-zombies are undead, zombie-like old people created by Grandfather to serve his every whim.
They serve as the secondary antagonists in the story Operation Z.E.R.O.
Senior Citi-zombie are a form of undead created by Grandfather, who can transform people into them through touch. When turned into one, a person becomes extremely old and decrepit to the point of being corpse-like in appearance, with other zombie-like qualities such as yellow pupil-less eyes.
Like Grandfather himself, Senior Citizombies can turn other people into their kind just by touching them. Victims of zombification do retain elements of their personalities, but lose their free will and become Grandfather's evil slaves regardless of who they are.
The first people turned into Senior Citizombies were the villains at the Convention Center, starting with Toiletnator. This already-sizeable army then proceeded to invade all the KND Treehouses, zombifying everything in their way in the process, until the entire world was under Grandfather's control, eventually even seizing the KND Moonbase. The people of Earth were only returned to their normal selves when Grandfather was defeated and decommissioned by Numbuh 0 and Sector V.
Age-o-tonic Birthday Suits were modified by Numbuh 2 to prevent zombification after the epidemic began, but were only partially complete by the time Numbuh 2 himself was turned into one. When Numbuh 1 put his team into the suits near the end of the film, their bodies are only partially covered and the parts that aren't are still zombified and still evil.
Members of Grandfather's bloodline have shown immunity to Senior Citizombie infection, including Numbuh 1, Numbuh 0 and Father.