Lowest inhabited islands
There are inhabited islands off the coast of Azerbaijan at 92 feet / 28 meters below sea level, in the Caspian Sea.

Northernmost island
The most northerly island is tiny Oodaaq Island off the north coast of Greenland

Oldest island
Science agrees that the oldest island in existence is Madagascar, which separated from India around 85 to 90 million years ago after the two had separated from Antarctica about 125 million years ago.
The first islands appeared on Earth about 4.4 billion years ago, when the oceans formed. However, most islands now in existence are extremely young, the result of sea level changes and glacial retreat over the last 12,000 years or so

Most islands in a country
Finland lays claim to the title of 'Country with the Most Islands' with a total of 179,584. The Canadians probably have far more but haven't got round to counting them all. Several small countries have no islands at all.