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.