I've been wondering about basic information about Rey, including how long she's been on Jakku.
I was thinking that this might also give us her age because she looked to be about four when her parents left her.
Rey's earliest history isn't described in any great detail in any of the canon novels or factbooks. We do know that after "several years" (and by the age of ten) she'd managed to scrounge her own shelter and we know that the actress that played young Rey Cailey Fleming was 6.5 years of age when she filmed the role. We have an unconfirmed article in Variety that places her age at five when she was abandoned but obviously none of this is sufficiently canon that it can't be subject to change.
We first meet Rey on the desert planet of Jakku, where she was abandoned by her parents at five years old, and it doesn’t take long before she’s swept up on an equally momentous quest alongside John Boyega’s Finn and the scene-stealing new droid BB-8. Variety spoke to Ridley about the surreal experience of joining the “Star Wars” universe, watching director J.J. Abrams turn into a “fanboy” around Harrison Ford, and her own star-struck moment meeting “Star Wars: Rogue One” lead Felicity Jones.
Her actual age at the time The Force Awakens takes place has been confirmed (in another answer) to be 19 years of age.
According to the data given in Valorum's answer, Rey would have been on Jakku about 14 years if she was 5 years old when abandoned and about 12 or 13 yeas if she was the same age when abandoned as the actress portraying young Rey.
As Valorum says there is no certainty about Rey's age when abandoned and new canonical stories might be published at any time to give her a different canonical age when abandoned. Since an older child would have a better chance of survival when abandoned, the writers of hypothetical canonical stories stating Rey's age when abandoned might want to make her time alone on Jakku closer to 10 year than to 15 years.
But who knows if those writers of hypothetical future stories will consider matters of plausibility.