Who is the world's oldest democratic country?

The United Kingdom is the oldest surviving democratic nation.

It partially depends on how democratic you need to be to count as "democratic".

Until the 20th century all allegedly democratic countries allowed less than 50% of the population to vote, because women were excluded. The UK started the 19th century only allowing landowners to vote, and gradually increased the franchise until all males had the vote.

Athens is usually quoted as the first democracy, but most of the inhabitants were slaves, the 'citizens' who could vote were a small minority.

In my opinion the country with the best claim to the longest running democracy is Iceland. Their parliament, the Althing, is the oldest one still in use. It was formed in 930 AD by Vikings.