Assuming the soul is untrapped and willing, and the creature is indeed dead, if a character casts Resurrection on the severed hand of a dead creature, what memories does that creature have (i.e. from the time it died or from the time its hand was lost)? What physical age would it be?
Inspired by comments on: How bad of bodily damage can Raise Dead cure?
You have a new friend. Assuming the "if its soul is free and willing" part (e.g. it isn't alive somewhere else in a body that is just missing a hand, or it isn't happy in the plane of its afterlife, or trapped in a Magic Jar or Soul Gem, etc.) you should get a complete person with all its memories and skills. Clearly, if you haven't done any research, it's kind of a crapshoot -- maybe your new friend is Albert Schweitzer, but maybe it's Vlad the Impaler.
Resurrection "... closes all mortal wounds and restores any missing body parts." The only requirements are that it be "dead for no more than a century", that it "didn’t die of old age", "isn’t undead", and "its soul is free and willing".
I would argue that a severed hand does not constitute a "creature." The text of resurrection reads:
You touch a dead creature that has been dead for no more than a century, that didn't die of old age, and that isn't undead. If its soul is free and willing, the target returns to life with all its hit points.
As a DM I would rule that a single small found body part does not constitute a creature, that it would have to be the most substantial part of the creature for the spell to work.
To clarify in the range of Resurrection spells:
At my table, I'd use the spells in the following ways:
Raise Dead - body exactly as it is, comes back to life.
Resurrection - substantial portion of a body, can regrow missing parts
True Resurrection - any portion of body or no body.
The text of the spell says it can regrow any "missing parts." I don't think of a hand as a creature missing an arm, torso, head, legs, etc., so I would say RAI of the spell is that it would not work on such an unsubstantial piece. If you could assemble a large amount of body parts together, I'd then agree to let them stitch back/regrow etc.
As the other answers show, there's some debate over exactly how intact the body needs to be for Resurrection to work. However, when the spell does work, it repairs physical damage to the body and returns the dead creature's soul to the now-healthy body. 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. You either resurrect 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. However, it's reasonable to assume that it's based on properties of the soul rather than the body (after all, that's what True Resurrection does). 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.
Not sure if this is how it works normally, but in my group it would bring back the creature and regrow whatever limb you're holding… that is, assuming there is a more intact corpse elswhere. Otherwise it would be the DM who has the say of weather it's just an animated body party or the full person. As a very powerful necromancer, the DM tends shaft me by only giving a living hand in such scenarios, which has led me down the path of making my own creatures with various limbs I find or collect.
