All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Religiously, there is no impediment for your grandmother to marry at this age, if there is no other impediment such as being in her waiting period or the like.
Besides, there is no upper age-limit in Islam for getting married as there are great and unlimited advantages in marriage. So, one should not look at the marriage from only one side, like enjoying sex or the like.
Since getting married is lawful for your grandmother, then you should not worry about this matter; rather, you should accept it.
Allaah Knows best.