It is a popular belief, but a fallacy, that we can learn about what contributes to extreme old age by studying very old survivors. This certainly needs to be done, but it is also necessary to know about the characteristics of other members of their birth cohort, in early childhood, adulthood and early old age. In this chapter we can say quite a lot about those surviving to the age of 90 and beyond, but in order to compare them with their contemporaries, who did not survive through earlier stages of the life course, we would need to undertake a job of reconstruction from data sources, many of which are inadequate.