Well in the comic, about the arm. Bucky actually loses his arm sacrificing himself instead of Cap by hurling himself onto a rocket headed towards the US or something. Can't remember where exactly but it ends up exploding on the way and gets found by the Russians still alive. I assume in the movie, he just loses his arm do to...I dunno frostbite or something due to the cold. Haven't seen the movie yet but guess i'll see tomorrow! Anyways, the Russians decide to brainwash him and re-purpose him as an assassin for their own country and inject him with a super soldier like serum. The thing is, the Russians put him in cryogenic freeze and placed in a warehouse. The comic described him as being like a toy you being out to play with, putting him away, and bringing him back out when you need him but yeah, they only bring him out whenever they need him for very very important missions, and that is why he has aged so little. I wonder how close the film follows this but I hope that helps!