If you receive an adult and it turns out to be temperamental/shy, it'll be a long time and a lot of handling before there are any improvements, if any at all. Juvenile snakes are scientifically proven to learn faster, and a juvenile received as feisty can tame down in a relatively shorter amount of time. As for a snake that's already tame, it doesn't matter if you receive it as an adult or juvenile, as it should remain tame unless you handle it incorrectly and roughly, (which I expect you wouldn't do) or has a sudden hormonal shift for whatever health reason. (unlikely) If you want a tame snake from the beginning with the best guarantee possible, your best bet is to spend time handling it before you buy it and observe its behaviors and reactions such as to movement or being touched. A few animals happen to be natural born cuddlebugs, and these are the best first pet snakes. (I lucked out with my first being placid. Isn't even head shy!)
Snakes, being as anti-social as they are, don't make solid attachments with their first owners like parrots, some cats, or genets do, and unlike them, they can be rehomed without much psychological hassle and can adjust to new owners relatively well so long as sound husbandry is practiced.