A juvenile in our county was adjudicated delinquent for Aggravated Sexual Assault and was placed on one year probation, with initial placement at Kerr back in 2003. He was never registered as a sex offender and there is no order excusing or deferring registration. (Prior probation officer and prior prosecutor, so we don't know why or how.) Has anyone ever gone back and registered a juvenile who "slipped through the cracks" years later?

Well your main problem is going to be under 62.053(c) or (d) and whether the juvenile was properly admonished.

If admonished properly by the court then you don't need an order, the adjudication requires that they register, an order is needed to defer or make the registration non public. If they just did not recieve the proper release notification then you can have the kid brought back in and do the proper notification and preceeded from that point.

If not admonished properly in writing or in court is going to be an issue that IMHO is going to preclude you from future prosecution of any failure to register case.