So the average fertility is up to 15, but the average life expectancy is 14. That makes no sense. "Some" individuals only set the standard if they live to be much longer to really have a influence on the standard and fertility all the way.
It's not like that 10% has to live 1 year longer. There must be some lions as old as Methusalem to influence the average. But like it says, the average life expectancy is 14, not 15.
I don't 100% understand what you mean by that but I will try to explain it differently.
Let's say a scientist did a study on how long lions live and when they become sterile. This might be his results:
Age | Amount of lions that have died at this age | Amount of lions that have become sterile at this age |
12 or younger | 10 | 0 (these lions all died before they could become infertile by old age) |
13 | 15 | 0 (these lions all died before they could become infertile by old age) |
14 (average life expectancy) | 50 | 0 (these lions all died before they could become infertile by old age) |
15 | 15 | 25 |
16 or older | 10 | These lions had all been sterile since they were 15 years old. |
Most lions die at 14, it's both the average and the median age of death. So the life expectancy of lions is 14. Of course there are outliers to both sides, some lions died earlier, others had a longer life. But none of the lions that died at 14 or before ever became infertile in their life. Only the lions that got older than 14 became infertile. And these older lions all got infertile at 15, so the age of infertility is 15.
Also note that it is assumed that an animal can not have offspring (in a natural way) if it's dead. "Infertile" only applies to animals that are still alive.

“Infertility is the inability of a sexually active, non-contracepting couple to achieve pregnancy in one year..." is a definition on the website of the WHO. Even though this definition is for humans, the "sexually active" part implies that both the humans or animals must be alive, because a dead animal or human can't be "sexually active".
Of course the real numbers are harder to work with and the numbers in this post were just invented by me to work as an example.