shes a Human/Time Lady, she can age as she wills, but also, The Silence could have transported her with a Vortex Manipulator.
- She escaped the Silence in "Day of the Moon". Mels did say in "Let's Kill Hitler" that she regenerated into a toddler and she also did say she was going to age in reverse. So it's possible she lived from January 1970 to ~1996 as a toddler and then aged herself to keep up with Amy and Rory.
Basically, it was never said on screen exactly how, however we can outline possibilities based on the evidence we have such as what's been stated above.