The Force Awakens takes place 30 years after Return of the Jedi, making Leia and Luke, despite his aged appearance, only 53 years old. (Being a Jedi in exile ages you; just ask Obi-Wan.) Ben Solo, born a year after the events of ROTJ, is 29. By all accounts Rey is 10 years his junior, making her 19.
Here’s where things get interesting: Keri Russell, rumored to be playing Mara Jade, is 42. At first people thought she’d be too young to play Mara Jade from the Expanded Universe. Keeping in mind Luke’s official canon age of 53 however, and the fact that Han Solo is ten years Leia’s senior, suddenly that age gap between Luke and Mara doesn’t seem so wide.
The timeline kicker: Depending on what source you read, Snoke sinks his claws into Ben Solo sometime in the boy’s late teens, when the Knights of Ren destroy the new Jedi Temple… right about the time one Miss “Rey” is born.