I've never bred Newfs, but with Goldens, it was as soon as they could walk around and were fully aware of their "world" (between 2-3 weeks). Some were more pushy, some more playful, some really feisty, some independent and adventurous, some calm, and some less adventurous ..and then there was always the bully who picked on everybody wanting to play rough, and would push the rest out of the way to get to the milk bar!