This DM rules that the spell simulacrum creates a creature that largely obeys the same rules as the original
A creature's type and subtypes tend to dictate whether or not a creature ages, whether it eats and drinks for survival or for pleasure, whether it must sleep or can sleep if it wants to or is forced to, and whether or not it breathes. A creature created by the 7th-level Sor/Wiz spell simulacrum [illus] (Player's Handbook 279–80) retains the original's type and subtypes, so all of these factors are duplicated from the original creature by the created creature, its illusory nature notwithstanding.
Whether or not the creature bleeds, though, is something else. The game has bleeding typically be a largely cosmetic effect, so this DM would rule that The creature that's created by the spell simulacrum bleeds if the original creature bleeds if for no other reason than such a creature's true identity would be too easily detected were it not to. To this DM, having the town guard cut everyone when they enter the city—and kill those who leak snow—sounds like it'd foil too many otherwise excellent plots. However, that also sounds kind of (ahem) cool—and maybe the spell is rare enough that such extreme measures are dismissed as paranoia?—, so this player wouldn't flip the table and leave the campaign of a DM who ruled differently.
The description of the 7th-level Sor/Wiz spell simulacrum [illus] (Player's Handbook 279–80) says that a the creature it creates "appears to be the same as the original, but it has only one-half of the real creature’s levels or Hit Dice (and the appropriate hit points, feats, skill ranks, and special abilities for a creature of that level or HD)." In addition, "A
simulacrum has no ability to become more powerful," the spell says later. "It cannot increase its level or abilities. If reduced to 0 hit points or otherwise destroyed, it reverts to snow and melts instantly into nothingness."
Further, D&D, 3rd Edition co-designer Skip Williams's oft-maligned Rules of the Game Web column in the column "Costructs (Part One)" clarifies the simulacrum spell in brief, saying, "The simulacrum spell creates a duplicate of some other creature and the duplicate has the same creature type as the original." (This clarification is particularly useful in light of this answer's opening!)
Finally, the most extensive source of information about creatures created by the spell simulacrum comes from the adventure Expedition to the Ruins of Castle Greyhawk (Aug. 2007) that sees one important foe actually being a creature that was created by the spell simulacrum that's attempting to gain a soul. In its stat block, this creature has the following entry:
Simulacrum If viewed with true seeing, [this creature] appears as an animate statue made of snow and ice. If reduced to 0 hit points or otherwise destroyed, [it] reverts to snow and melts instantly into nothingness. (214)
The creature's stat block is otherwise normal for the era.1 While a major force in the adventure, this creature is not—as is the case in many published adventures—the product of some unique magic. On the contrary, the adventure describes the creature's creator as simply having found a strand of the original creature's hair then using that hair as the material component in a scroll of simulacrum.2,3
Thus this reader is led to believe by Expedition that the only differences between the original and the creature created by the spell simulacrum is that—in addition to all that halving—the creature lacks a soul, it's probably not supposed to regain lost hp naturally or magically but instead by the method described in the spell, and it can't gain (but it can lose) XP. In other respects, the creature created by the simulacrum spell is the original creature.4,5
1 Okay, not entirely. The creature's type line includes between its race and class the word simulacrum and its type is humanoid (simulacrum). Simulacrum as a subtype? Sure. Whatever. Also, the creature inexplicably has immunity to mind-affecting effects, the source of which is unmentioned by the stat block. This reader suspects that this could be due to the creature being the product of the spell simulacrum, but this is otherwise unsupported, wholly unmentioned elsewhere… including, so far as I can tell, by the adventure itself!
2 That scroll of simulacrum had a price of at least 11,625 gp. So you know.
3 This excess of carelessness on the part of the original creature is just one more reason that every adventurer should shave his or her head. I'm not even kidding.
4 The creature in Expedition has access to superior resources yet maintains a laboratory stocked for self-repair (186) and leans heavily on the 2nd-level Sor/Wiz spell false life [necro] (PH 229)… presumably so it needn't use that lab! Further, the creature's Special Qualities entry includes familiar with this note: "[its] fear of losing [its] precious experience points has prevented [it] from using this class feature" (214). Finally—and I'm not making this up—, the creature's drawn from a deck of many things (Dungeon Master's Guide 278–9) (minor artifact; 0 lbs.) the card Star, gaining a +2 inherent bonus to one ability score, implying that when the spell says A simulacrum has no ability to become more powerful means It can't earn XP rather than It can't gain power even in some asymmetrical other way.
5 Other published uses of the spell simulacrum aren't nearly as detailed. For example, the Web adventure "The Thunder Below" (2004) sees a stealth nerf to the spell simulacrum that's never followed up on in later texts: "Since simulacrums have no memories in common with their source, though, their immediate usefulness as informants is limited" (9). The frightening creature at the center of the online-only Dungeon #153 adventure "Prisoner of the Castle Perilous" (2007) doesn't mention it possessing any of the features of the subtype (?) simulacrum, and "Legacy of the Crystal Shard" (2013(!)) sees a villain using (sigh) a unique-to-that-adventure version of the spell simulacrum.