The evidence regarding life-shortening is derived mainly from animal experiments where radiation has been demonstrated to shorten lifespan.

The aging process is complex and largely obscure; the exact mechanisms involved in it are yet uncertain. Irradiated animals in these investigations appear to die of the same diseases as non-irradiated animals, but they do so at an earlier age. How much of the total effect is due to premature aging and how much to an increased incidence of radiation-induced diseases is still unresolved.

However, data from the populations of Hiroshima and Nagasaki indicate that, if life-shortening occurs it is very slight, less than 1 year per 87.7 rad.

Select this link for a table showing typical latent periods between exposure and effect.