They are easily recognized by a red dot above their left eyebrow. They are normal human beings until they reach the age of thirty, at which time they become dejected. Upon reaching the age of eighty they become legally dead, and suffer from many ailments including the loss of eyesight and the loss of hair.
Struldbrugs were forbidden to own property:
As soon as they have completed the term of eighty years, they are looked on as dead in law; their heirs immediately succeed to their estates; only a small pittance is reserved for their support; and the poor ones are maintained at the public charge. After that period, they are held incapable of any employment of trust or profit; they cannot purchase lands, or take leases; neither are they allowed to be witnesses in any cause, either civil or criminal, not even for the decision of meers and bounds.
Because:
Otherwise, as avarice is the necessary consequence of old age, those immortals would in time become proprietors of the whole nation, and engross the civil power, which, for want of abilities to manage, must end in the ruin of the public.
The term struldbrug has often been used in science fiction, most prolifically by Larry Niven, to describe supercentenarians.