The law is clear: dynasts were Königliche Hoheit from birth. That is evident from state almanacs, but also from newspapers. Consider this news story about Prinz Rupprecht when he was seventeen years old:
https://digipress.digitale-sammlungen.de/view/bsb00085485_00703_u001/17?cq=%22hoheit%20prinz%20rupprecht%22 It is possible that PRACTICE was not to use these styles at court when princes were young. But I'd want to see better evidence of that.