I think the younger the snake the easier it is to tame it down, mainly because I think it's easier for people to take a bite from a small snake than a large snake. When I was a kid we used to catch grass snakes in Wisconsin, the small ones never bit and if they did they didn't even break the skin. The larger ones would draw blood, some actually would surprise me they were so aggressive. But all of them tamed down with handling, mostly the smaller ones got super tame. The bigger ones sill had that wild streak in them even after we tamed them down. I couldn't imagine trying to tame down a mean 12 foot reticulated python! But when they are small it would be much easier.
I also think a lot of it has to do with the breed of snake. Some snakes that tend to be mean seem to always be harder to tame down, like rat snakes, green tree pythons, and blood pythons. However, I have seen people tame down mean breeds of snakes (even green anacondas), I think it just takes working with them when they are younger and more handling, building the trust between man and snake.