The US has been servicing debt since 1837, and since the US national debt have never been fully paid off since then, I suppose there could be some debt that have been continiously serviced since then.

National debt is a complex issue (and outside of my expertise) and simply paying off the debt have rarely been an option. When GDP rises, so does tax income. Money that could have been used to decrease debt could be better used invested in infrastructure, healthcare and so on which allows the citizens of a country to be more productive longer and thus pay more taxes

I know a few years ago that the government revealed it was still paying a number of pensions to veterans from the Spanish-American War. Are there any other historical--the past is not even past-like--commitments?

we are still paying off civil war pensions because those went to wives and children. Ms. Triplett

http://www.wsj.com/articles/SB10001424052702303603904579493830954152394

That's the earliest of those types of obligations. As others have mentioned the us doesn't have anything as fun as Britain's 18th century debts

I would just like to add that Debt in America is also branched into Federal and Municipal (note I am not a historian just some knowledge about MuniBonds).

However, here are some of the answers to your more specific questions.

Are there any other historical--the past is not even past-like--commitments?

Yes, because of the nature of a bond it needs to be REDEEMED. So while technically it is considered "paid" if there is a "refinancing" (literally like a mortgage, a state can refinance up to 3 times i believe) that doesn't mean the debt has been collected.

Example: Bearer Bonds, or US Bonds that haven't been cashed. Example of the type of information you are looking for...http://usatoday30.usatoday.com/news/washington/2011-07-21-debt-limit-history-old-debt-George-Washington_n.htm (more complex) notice they state they are "no longer receiving interest" this generally counts as paid on a govt balance sheet.

Why do these debts last so long? Debt is nothing more than a contract; Usually Bonds last 5-30 years; Notes 18months or less usually, etc.

Someone wrote them to last that long, no different than a mortgage. Other times they might let the debt "survive" because of a lower interest rate. (this will be addressed in your next questions)

Who holds them?

Mostly residents of the USA, lots of companies, rich people, banks, blah blah.

Wouldn't it be cheaper for countries to pay them off sooner?

MAYBE.

If you look at interest rates historically they have spiked and plummeted multiple times. If you have a high interest bond (which means not many people want to buy it) then it is in your best interest to pay it off sooner rather than later.

If you have a bond like a US Treasury today that has an interest rate ("coupon rate") of something like 2% per year then you wouldn't pay this off until you need to pay it off.

If the debt is "no longer generating interest" then for all intent and purpose the debt has been "paid" (somewhere there is an escrow account making sure that money is safe. This would no longer be considered "debt".

Wouldn't it be cheaper for countries to pay them off sooner? PT2

Paying something off EARLY is called an early "redemption" or a "mandatory call". It needs to be CONTRACTED originally and known at the point of sale to customer. At this point as stated it is no longer "debt" as we consider debt.

BONUS FACT - a debt that is NOT paid in time is listed as "DEFAULTED" even if the payment is late one day. This allows investors to go to court (if in their benefit) to "accelerate" a debt. Conversely, if a Bond or Debt has never had a "default" in payment then it considered paid on time.