For some of these I have illustration, for others not (haven't photographed everything yet).
The oldest coin in my main, British, collection is a penny of Edward I, 1272 -- 1307
The oldest coin in my "Russian" collection is a dirham from Bukhara (now a part of Uzbekistan, former Soviet republic), 775-785 AD
My oldest (proper) Russian coin is a denga of Ivan the Terrible, circa 1550.
My oldest European, and oldest overall, is a Roman AS of Trajan
, AD 115 -- 116, a gift from Scooby (I guess he regifted it
My oldest Asian is an Indian coin from the 5th century (I still need to research and ID it)
My oldest African is a pysa of Zanzibar, dated AH 1299 (AD 1881)
My oldest American coin is 2 sous, 1789, from French Guiana. (My oldest US coin is 2 cents 1865)
My oldest Australia/Oceania coin is an Australian shilling of 1910.