The example: if the average death age in 2006 is 80.
Then if you can go back in time thru a time machine, you can go back and predict that anyone born aounr 80 years ago to expected to live 80 years old.

For babies born in 2006, you do not really have data
to estimate, but you have to use historical data
becaus eyou have no choice. So life expency even
for 2006 born babies is till 80 years old
We can intuitively expect them to live longer than
that because new technological advamce in medcien can be made but you really have no data yet.