That's exactly what happens.

The life expectancy of a person who lives to 18 increases dramatically across the board.

You even see this today. If you live to 70, you can expect to live 14 to 17 more years. So to about 84 on the short side and 87 on the long side.

However, make it to 75, your life expectancy is 11 to 14 years. Which is 86 on the short side and 89 on the long side.

Basically, the longer you live, the longer you should live, but eventually the range collapses. And that's because people come in under the average all the time. Some people die at 71. And those people stop getting counted when determining life expectancy at 75.