The US has never demonetized coins as far as I am aware.
My oldest "easily spendable" coin is a 1906 US Indian Head cent. I could spend that tomorrow and probably nobody would even look at it twice, being the same size and color as the current cent coins. And I have a 1910 wheat cent that definitely wouldn't attract any attention, since the obverse looks just like a modern coin. I have a 1900 US quarter (Barber design) and if I spent that it would probably get a second look but would be accepted (and potentially gleefully pocketed by the cashier if they realized it was silver).
I have a 1906 Swiss 5 rappen, too, but that's not as easy for me to spend from the US.
Oldest probably technically legal tender: 1699 England 4 pence (it is somewhat debatable how far back does the legal tender status of Maundy money go - some sources say only to the 1820s, some say it's the entire run all the way back to Charles).
Oldest I know is legal tender: 1820s (date worn away) US large cent (as mentioned above, aside from some very specific exceptions, no US coins are currently demonetized).
Oldest that's legal tender and sufficiently similar to a modern coin to maybe pass by in circulation (i.e. has the same size/color as a currently circulating type): 1864 Indian Head cent (it's the cupronickel variety, though, so it's thicker - if that doesn't count, the 1866/7 dateless shield nickel with rays)
Oldest that could actually be spent without being noticed by anyone who doesn't look at the dates (i.e. has the same design as a currently circulating type): Swiss 10 rappen from the 1890s (forgot the exact date)
Oldest that I could probably spend in my country without anyone noticing: 10 kopek (and perhaps 1 ruble) 1991 (I've actually received both of those types in change a few times - but they're technically not legal tender)
Oldest i recall having ever actually spent: 1985 (forgot the local date) Israeli new sheqel
Note: some of those coins have not been entered on my Numista account so far.
The US has only one type of coin demonetized , gold coins. The only US coins not legal tender. To my thinking the US has never re-monetized any coins. But this is the oldest coin I got in change. A 1903 V nickel
And I found this in the parking lot at the store
So they were used so I think I can use them now
In New Zealand, we have only had our own coins since 1933 and we changed to dollars in 1967.
To make matters worse we eliminated several coins and their sizes between 1989 and 2006.
The oldest spendable coins now are these
Yep just a 1990 $1 and $2 coin, first photo shows a pair as new, 2nd shows what most look like now (Ignore the 20c coin, it was demonitised in 2006).
But if we go back to 2006 with the old 5c, 10c, 20c - they were the same size and value as old 6d, 1/-, 2/- although any from 1946 or older were half silver and be worth well above their face value. 1947 - 1965 predecimal coins circulated easily with these. But the 2006 coin shrinking exercise made all these coins invalid.
The oldest coin I ever got in change was in 2005, when I got a very worn 1942 1/- coin as a 10c in change. I was happy as it contained about $2 worth of silver then ($5 now).
Before 1933 we used British coins and thus any coin back to 1816 was legal tender as an 1816 1/- coin was the same size and value as a 2005 10c piece!
I love coins. Especially silver, gold and anything really old.
Member of the Royal Numismatic Society of New Zealand and the Auckland Numismatic Society