Is liver just liver or is there some basic difference in beef, swine, chicken. If I take a liverwurst recipe and use chicken liver instead of pork am I gong to get the same results within + or - 10%. Is liver from a shoat the same taste as a 60 kg sow?
Let me do a breakdown of the typical ingredients of liverwurst ("Leberwurst") roughly based on food laws in Germany (aka liverwurst country):
- roughly 10% - 30% (sometimes up to 40%) liver:
mostly pork because it's cheapest, using partly veal or poultry is more expensive but tastewise no big difference.
- muscle meat and bacon, again typically pork, but beef, veal and poultry is also used either alone or as add-in.
- usually a generous amount of spices.
The liver is influencing taste and texture, but the used regular meat and especially the spices play a big role, too. Using a "too dominant" liver or too much in total can be overwhelming, but there's no accounting for taste. While pork, veal and poultry livers are comparatively "mild" and can be used interchangeably (or as you said, the results will be in your +/- 10% range), beef liver has a tendency to make the liverwurst bitter, so it's use is not recommended.
Every recipe and article I've seen implies that there are only two options for traditional, German-style liverwurst.
- Pork (more common)
- Beef (usually calves' liver)
Liverwurst usually contains pigs' or calves' livers. Other ingredients are meat (notably veal), fat, and spices including ground black pepper, marjoram, allspice, thyme, ground mustard seed, or nutmeg.
That being said, liverwurst is, essentially, a type of pâté (the French term)... of which the most iconic is, arguably, goose liver pâté or "pâté de foie gras":
The most famous pâté is probably pâté de foie gras, made from the livers of fattened geese.
Other regional versions include chicken livers:
In Russia and Ukraine, the dish is mostly prepared with beef, goose or chicken liver and thus is commonly known as печёночный паштет (Russian, pechyonochniy pashtet), however other meats also can be used.
So, it seems that it's up to you to decide what liver you want to be based on your taste preferences and how traditional you want to be.
As to your secondary question, none of the articles I could find specifically mention the age of the pig, though it's common to use veal liver if using beef instead of pork.