I feel like writers have been inconsistent on how Mystique found her
Scenario 1 Rogue ran away from home very young and Mystique took her as in prepubescence. The Cody kiss happened while already living with Mystique and Destiny. Could have been living with Mystique anywhere between 6-12 years.
Scenario 2 Mystique took her in after the Cody kiss as an adolescent, as soon as her powers manifested. Only lived with Mystique during her teenage years, anywhere between 3-5 years.
I like the first scenario more, I think it really emphasizes that Irene and Raven were her mothers rather than just mentors.