"The Luggnaggians commended. A particular description of the Struldbrugs, with many conversations between the author and some eminent persons upon that subject."
Gulliver finds the Luggnaggians pretty nice overall.
One day, he's chatting with some locals, and one of them asks him if he has seen "any of their struldbrugs, or immortals" (3.10.2).
Every now and again, a child will be born with a mark on its forehead, over its left eyebrow, which shows that it will never die. These are the struldbrugs.
Gulliver is really excited to find a country where every child has a chance of being born immortal.
The person Gulliver is speaking to asks Gulliver what he would do, if he had been born immortal.
Gulliver jumps in: he would make lots of money, invest it and save it, become the wealthiest man in the kingdom, learn everything there is to know about everything, and write down all the events and fashions that he sees to provide future knowledge for the nation.
Gulliver would also take care to instruct young people, but most of his friends would be fellow immortals, since what would be the point of hanging out with lots of people without the benefit of his experience?
Gulliver goes on to exclaim about all the discoveries he and his struldbrug friends would make – it would be amazing.
The person Gulliver's talking to tells him he's being an idiot: in fact, the terrible thing about being a struldbrug is that you are immortal but you are not eternally young.
The struldbrugs age at the same rate as other humans, the difference being, that at 80 years old, they're much more miserable than other old people because they have the prospect of living on and on beyond their 80 years.
As soon as a struldbrug turns 80, he is dead in terms of the law, so all of his money goes to his heirs – he's totally poor.
Struldbrug marriages are also dissolved at 80, since they would make the couple so much more unhappy.
At 90, they start losing their teeth, so they don't enjoy eating anymore.
Their memories get bad enough that they can't read without forgetting, at the end of a sentence, how it began.
Because language evolves with time, older struldbrugs can't understand younger people at all.
They have to beg for money, since otherwise, they must get by on a tiny state allowance.
Gulliver feels ashamed of wishing to be a struldbrug, since being one is so completely awful.
At the same time, the Luggnaggian King does remind him that the sight of a struldbrug cures everyone of fear of death.