In the winged insects, moulting ends at the adult moult, because a functioning wing cannot be shed and regrown. Silverfish belong to the Zygentoma, a lineage that diverged before wings evolved, and they retain the ancestral pattern of moulting throughout life.
The practical consequences are unusual. A silverfish that loses a leg or an antenna can regenerate it across subsequent moults, and repair of cuticular damage is available to the adult rather than only the juvenile. Adults may moult many times, with lifespans reported in the range of several years, longer than most household insects. Development is ametabolous: hatchlings resemble small adults and there is no metamorphosis.
Diet explains the pest reputation. Silverfish digest polysaccharides including cellulose, aided by gut enzymes, so paper, bookbinding paste, wallpaper and starched fabric are food. They need high humidity, which is why they concentrate in bathrooms and basements, and drying out an infested space is more effective than most direct treatments.