The books never mention how the Trace is applied to young witches and wizards, and Accio Quote doesn't turn up anything relevant from interviews with Rowling. Any answer would have to be speculation. So I'll speculate. I see two parts to this: how the Trace is applied, and when. Let's start with how. Nobody who talks about the Trace in the books -- neither Moody, nor Ron or Hermione -- ever actual
![How is "The Trace" applied in Harry Potter?](https://cdn-ak-scissors.b.st-hatena.com/image/square/dc0465e66e375a28d22f1f5d282d40b0cfacfd73/height=288;version=1;width=512/https%3A%2F%2Fcdn.sstatic.net%2FSites%2Fscifi%2FImg%2Fapple-touch-icon%402.png%3Fv%3D21318a38b766)