[@AnnaBeth]He would have been the youngest child, and possibly the only one that was Force Sensitive.

He could be from a world outside the Republic, where the Jedi wouldn't have tested everyone at birth.  The world would have allied with the Republic because the Seps tried to conquer it.
Or maybe he wasn't quite up to the Jedi standards so they didn't bother "recruiting" him as a baby. Or he was born outside the Republic, but his family moved to a Republic world after he was born, so the initial testing wasn't done.

We do know it's possible for people to slip through the net, as Palpatine wasn't detected despite being born on Naboo.

Edit: Let's just say he was from a low population planet (a few million) and that when the doctor told the parents what the at-birth midiclorian count test meant, they bribed him to falsify the results by sending a duplicate of his older brother's much lower count to the government.