All Greek citizens aged 18 or over in the year of the election are eligible to vote. Compulsory voting is the law in Greece but is not enforced. In the past a citizen had to present an up-to-date election booklet in order to be issued a driver licence or a passport, or else justify why they did not vote (e.g. because of absence, infirmity, or advanced old age).
Nowadays the civic duty of voting is still considered “mandatory” but there are no sanctions for failing to vote. Turnout is usually high, typically between 70 and 80% for legislative elections and slightly lower for local administrative and European Parliament ones.