It used to somewhat work for humans, until about the mid-19th century (in Europe, other places vary). A newborn was unlikely to reach an age of 5, a 5 year old had better chances but might still die from illness in childhood, but someone who reached an age of 20 could be expected to reach an age of 40, and if you reached 40 you might well make it to 80.

99 year old humans are considerably more likely to live to be 100 than 50 year old humans.