In the episode A Good Man Goes To War the Silence take Melody Pond as a baby, and in Let's Kill Hitler she is tan and in the form of Mels before she regenerates into her current form.
What caused her to regenerate from her original form to Mels?
Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.Sign up to join this community
We aren't told. When we last see her as a young child, she is in a back alley. She indicates she is dying (possibly from being shot by Amy) and with a burst of regeneration, the scene ends.
She could have also been sick from living on the street, she seemed little more than a ragged orphan at that moment.
When we next see her, she is hanging around with her parents Amy and Rory as children. We are not told how she ends up in school with Rory and Amy. She appears to grow normally from that point on, in sync with Amy and Rory and appears to be close to the same age as they are.
Either she was put there by the Silence (as a means of keeping close to her parents when the Doctor would eventually return.)
Or she was put there by the Doctor to keep her safe and available (keeping your friends close and your enemies closer; neither answer really satisfies me). Its one of those things we are just told to accept on faith.
We are lead to assume somewhere in there, she was being brainwashed and conditioned by the Silence and that her conditioning was able to survive her regeneration into the adult River Song identity.