Radioactive iodine for hyperthyroidism?

I read online that radioactive iodine was one of the treatment options available for hyperthyroidism, but i read somewhere that if you are under the age of 20 or 25 you shouldn't take it. Is this true? At what age is it ok to have radioactive iodine as a treatment for hyperthyroidism? I thought that it only affected your thyroid and was safe. Please answer if you know! Thanks!