The incident occurred last night, they said.
The woman, who could not speak properly and had weak memory, used to go a nearby religious place to offer prayers.
Yesterday evening her son noticed that Raghbir Singh, son of Bhagat Ram of Haler Janardhan, was sexually assaulting her in a forest area, the police said.
On seeing him, Raghbir fled.
Police have booked the accused.