As the other answers show, there's some disagreement over exactly how intact the body needs to be for Resurrection to work. However, when the spell does work, it does so by repairing physical damage to the body and returning the dead creature's soul to the now-healthy body, so the resurrected creature has all of the memories of the dead creature (including whatever memories it keeps from the afterlife, though exactly what memories are kept is a frequent subject of setting variations and house rules).
Because it uses the creature's soul, there's no way for the spell to create a copy of the original creature with some of its memories, if that's what you're imagining - you either get a resurrected version of the actual creature, or the spell fails (e.g., because you were trying to resurrect the severed hand of a still-living person). Even if the hand was severed before the person actually died, they have their full memories, because you're just using the hand as a link to the soul, which is what actually keeps track of the memories.
As for physical age, the spell and others like it are mostly silent on this topic, but it's reasonable to assume that it's based on properties of the soul rather than the body like other parts of the spell. In this case, that means the spell creates a new body that mirrors the one that the soul inhabited up until it was slain and its soul moved on to the afterlife, including age. This does mean that if you were to resurrect someone using a hand that was severed 20 years earlier, the hand would (in a sense) age 20 years in the process. If that bothers you, I think it would be reasonable to rule that severed body parts lose their connection to a still-living soul over time and don't work for the spell past a certain point.