I would define "immortality" (i.e., with the quotes, so not immortality) as the ratio of average or mean life expectancy to years-to-maturity. Today our "immortality" is ~4.4. If we lived to ~200 years with adulthood still starting at 18 we'd have an "immortality" of ~11. If we lived to 1,000 with adulthood starting at 18 then we'd have an "immortality" of 55.56.
We could fix these problems by lengthening childhood by the same factor as life expectancy, but then the new thing wouldn't be very interesting.
I would define "immortality" (i.e., with the quotes, so not immortality) as the ratio of average or mean life expectancy to years-to-maturity. Today our "immortality" is ~4.4. If we lived to ~200 years with adulthood still starting at 18 we'd have an "immortality" of ~11. If we lived to 1,000 with adulthood starting at 18 then we'd have an "immortality" of 55.56.
We could fix these problems by lengthening childhood by the same factor as life expectancy, but then the new thing wouldn't be very interesting.