The type of handling is important, not just handling. Bad handling will make a snake more likely to bite, not less. Most pythons which are never, ever handled, are good handlers. Wild snakes are bad handlers because they have experience with a world where any large animal which approaches wants to kill you.
Captive snakes which are never handled see humans as harmless parts of the environment. They don't attack, they come, they go, they are seen often but never cause trouble, so the snake is comfortable with them and don't see them as threatening. If you then pick them up they're usually fine. If a snake is scared and you pick it up and it's striking at you, especially if you restrain it as all, it thinks you're trying to kill it. When it bites and bluffs and tries to scare you, when you do put it down, it assumes you wanted to eat it but it managed to scare you, or assumes when it bit you it injured you and you gave up. So, next time you pick it up it will assume it again needs to fight you to avoid being eaten, and this can become an established pattern. Many newbies are told to handle frequently so the snake gets used to it without any further advice, and when it develops into this negative cycle they become very frustrated and disheartened.
Some snakes are just lovely, others will always be grumpy, most can be changed depending on how you treat them, but once you've turned them nasty it can be difficult to turn them nice; it's easier to gain trust from the start than to regain trust after losing it, and a snake won't understand that you never meant to hurt it, and they don't actually want or need to be handled.
Each snake is an individual. I don't know your snake but most likely at that age if it's handling badly, the first step would be to not handle it for a while, then very slowly introduce handling (depending on the snake this might start with just touching it gently when cleaning the cage, stroking it with a hook, just holding part if its body briefly without picking it up, etc. Honestly, it's something I definitely just wouldn't bother with!). Just picking up a snake which is scared of humans and forcing it to be handled for 5-15 minutes will often be counterproductive. Don't handle until it's completely comfortable with you being nearby without touching it.