While I agree 100% with tex that this bird is plenty young and has plenty of time to change It's worth looking at the other side of the coin: He may change, but not necessarily for the better. There are many stories of people bringing home the sweetest bird in the store that latter turns into a devil, so be very cautious about bringing home a bird that has already displayed aggression.
That being said more observations are probably necessary. Is he actually "attacking" your finger, or is he just exploring? Birds explore the world with their beaks; even the most friendly socialized bird with explore a new finger in his cage with his beak.
Secondly, what kind of environment is he in in the store? What kind of cage? How often is he let out/handled? Does he let the workers there take him out or does he nip at everyone? (dont take the employees word for these questions, OBSERVE for yourself)
The answers to these questions will show the difference between a well raised bird that simply has no reason to trust YOU yet, versus a poorly raised bird who will be difficult to work with. [Any bird can be socialized by experienced handlers, but if this is your first you don't want a "fixer-upper"]
There was one local "pet store" here that had a horrible amazon. I put the term in quotes because I don't think they qualify; they were an outlet for the puppy mills and their care bordered on abuse. They had a large amazon in a SMALL cage that would be hardly appropriate size for a cockatiel. He had ONE dowel perch, a food and water dish, and no toys. The cage was attached to the ceiling, so he was far above anyone's head an he was never let out of the cage. The store put a sign on the cage "do not put your fingers in the cage, he bites!" I saw that sign an thought 'no $H!T he bites.' I really feel sorry for that bird, but I'd also feel sorry for any inexperienced owner that would take him home.
This tragedy is the minority. I've seen many other pet stores treat their birds quite well, socialize them, take them out... In these good stores many birds still need to get to know you before they trust you.
Somewhat counter-intuitively its been the large chain stores in my experience that have treated their birds well (ie Petsmart) while its the small locally owned places that I am generally more cautious about.