To protect workers and the public from the harmful effects of radiation exposure, it is important that regulatory bodies and advisory organizations have as complete an understanding as possible of the risks according to gender, age at exposure, time since exposure, health status and other related variables. The 2011 disaster at the Fukushima Daiichi nuclear power plant complex again alerted the world to the possibility that large groups, including many adults, can be exposed to ¹³¹I, it reminds us that it is important to understand the effect of age at exposure on cancer risk to achieve effective radiation protection and to plan for responses to future nuclear accidents or terrorist events involving radiation.