In the Ja'fari school of fiqh, there is no iddah (of divorce) upon a girl who is non-baligh nor upon a woman who has entered menopause. The verse in Surat al-Talaq has an important clause that deserves attention - if you doubt - and in various tafasir, it is used to refer to women who have reached the age of menopause, yet the menstrual cycle is not ascertained to have stopped definitely. In such a case, the woman, even if old, would have to keep an iddah of three menstrual cycles. The other case is of women of reproductive age, who are experiencing amenorrhea, and hence they have to also observe iddah like normal women their age would. Also if a man never had sexual intercourse with his wife, regardless of her age, she does not have to keep an iddah.
This is the link to fatawa from Sayyid al-Sistani concerning the above:
In the case of iddah of death, every widow has to observe a set four months and ten days, unless she was expecting, in which case she will wait out the term which is longer.