Like most breeders I sex them shortly after hatching. Vets are generally extremely bad at sexing, but if you get one of the good ones they'll be able to sex Carpets at any age. If they can't sex them at any particular age, I wouldn't trust them to sex them at any other age. It's a bit unusual these days to buy common snakes like Carpets unsexed.
Bredli are usually very calm. With most snakes if they are still snappy at around 5 months they'll probably grow out of it, but if you get a snappy bredli it's likely to always be a bit snappy, or at least be more difficult to calm down. Force handling is usually counterproductive, but it's what most people will tell you to do. Generally, anything which freaks them out (if they're fear biting you it's because they're terrified of you and think you're going to eat them) is going to be counter productive, especially if it includes physical interaction. Basically, if you pick it up and it's biting you until you put it down, it thinks you tried to eat it but it managed to scare you off, and next time it needs to do the same thing, but unfortunately this is what most people recommend, which of course leads to a lot of frustrated keepers and frightened snakes. Limit interaction to whatever doesn't scare it enough to make him try to bite. If he's striking at you from inside the tub when you walk past you have a pretty terrified snake on your hands, and I definitely wouldn't attempt to handle it until you can do things like change the water bowl without it trying to bite you. Picking up a snake like that is just going to traumatise it. Just with normal maintenance you'll be giving it more interaction than is ideal for such a snake.
Generally speaking, snakes hatch out thinking everything is probably going to try to eat them (which in nature is more or less true) so they try to scare everything away if they can't remain hidden. Over time, they learn that some things aren't dangerous, like, those big bouncy things (kangaroos) just hop by and literally never try to attack, so they stop being scared of them. Wild snakes near walking tracks which remain unnoticed often even becomes comfortable with humans and just sit there while people walk past them, or in private gardens where it's always the same people pottering around in the garden, this is pretty common. But a wild snake which has never seen a human will almost always flee if it sees one, and many of them will try to bite if picked up. Try to increase the amount of interaction slowly, and try never take it to the point where the snake is scared enough to try to bite you. An unnecessary trip to the vet is just an expensive way to scare the snake, get told everything is fine (which any quarter decent keeper can do) and possibly pick up a virus or something (vets are literally where people take their pets when there is a problem, and vets aren't even allowed by law to sell snake medication for snakes with contagious diseases without the snake being brought in; I'd personally never take a snake to one and would only ever recommend them as an absolute last resort, never ever as a routine thing).
