matty_022001,
If you are asking when a snake can physically breed, some can easily under a year old. That is ONE reason why it is not advisable to house snakes together, you don't want any accidental breeding of a young female since this could cause egg binding and death.
If you are asking when a snake can be bred. The snake should be at least 2 years old, 36" and 300grams.
And, as far as why the young one tries to jump out of your hand and the adult doesn't, it is because neonates are programed for self preservation which means getting away from the monster that is going to eat it. You are a preditor in its little mind. Once the baby grows older and is held often, it does not perceive being held as going to be eaten so is not overly worried about it. Some babies "tame down" quicker than others but for the most part, all of them eventually get there. There are, of course, always the exceptions to all rules but they are not the norm.
Good luck with your new addition.