You can have a 100-year extension to your life…
…but you have to spend that 100 years in a luxury New York penthouse overlooking Central Park.
The 100 years starts after you die at whatever age you die at and you remain that age, physically and mentally, for the duration.
You cannot commit suicide. Food and clothing will be provided to a minimum level and, if you take a job working online remotely, you can earn more money to buy nice things. If you already have money you can use that money to buy things.
Rent and utilities are paid. You cannot make any substantial changes to the physical architecture of the apartment. You can change sofas, dishwashers, etc for new or different models as the years pass.
The penthouse has a gym and a large bathtub and shower, but no pool. It has a small terrace meaning you can go outside and get fresh air, sit at a table and look at the park. You have a few plants. You can’t jump off the terrace.
You can have people come and visit you at the apartment, you can host parties as well. You have access to TV, movies, video games, and the internet. All mod cons.
You can marry someone (the wedding and reception would have to be in the apartment), and they can live with you and come and go, but you cannot leave the apartment for 100 years.
During that time, the building will not change or move. The neighbouring skyline will change, but you’ll always have that view of the park.
You won’t have a long decline. On the final day of the hundredth year, you will just cease to be and whatever happens next happens. Someone you care about will deal with your remains. If there’s no one, we will deal with them.
Finally, if portal technology is invented in the next 100 years, you specifically may not use it to travel somewhere else, even if still technically within the apartment.
CLARIFICATION: You will be a healthy version of the age you die. If you are 99 and die from agonizing emphysema or cancer, you will not have those things in the penthouse.
You may have the limited mobility and cognitive function of a 99-year-old, but you'll be healthy. If you die at 35 in a horrific accident where you are dismembered and after two weeks pass away, you will have all your limbs restored in the penthouse and no physical pain beyond the awful memory of the accident. If you were born blind, you'll be able to see. Deaf, hear. If you were born parapalegic, you will be able to walk in the penthouse.
The point is not for you to live in physical pain as a trade-off. The challenge is the restriction in movement for 100 years and any ensuing boredom that might come with it.