Why is the iddat for a widow, for 4 months and 10 days, compared to 3 months/periods for divorce (talaq)? And will there be an iddat in either case, if the lady has menopause or is too old?
The purpose for iddat is to clarify beyond doubt whether the divorced or widowed wife is pregnant or not. This purpose is clear from the fact that iddat finishes as soon as a pregnant wife gives birth to the child (Quran 65:4) and the fact that the wife who hasn’t physically met her husband has no iddat (Quran, 33:47). The apparent reason why there is a difference between iddat of a divorced and widowed woman is that since the purpose in both cases is to establish pregnancy, and in divorce, the husband has been advised to give talaq when the wife is not menstruating, waiting for three more clean periods establishes the matter beyond doubt. In case of widows, the time of death is not known and that clarity cannot be established with certainty. The duration has therefore been extended to not allow any room for doubt.
Going by this understanding, if it can be established with complete certainty that a woman is not pregnant, she will not need to go through any iddat. And Allah knows best.