In Athens the right to participate in the political life of the
polis was limited to what we would consider a narrow segment of the population. Participation was restricted to free
adult (18 or older) male citizens, thus excluding women,
resident aliens, and slaves.
In practice, this limited participation to perhaps 10-20 percent of the population.
In 451
BCE, Pericles introduced a law further limiting participation,
by providing that one was an Athenian citizen only if both of
one’s parents were citizens; previously it was enough if one’s
father was a citizen. A person’s citizenship was determined
by reference to the Assembly List of his deme.
A citizen could lose the right to political participation as a
punishment for certain offenses, such as owing a debt to the
public treasury, prostitution, beating or failing to support his
father or mother, throwing away his shield in battle, or
squandering his inheritance.
http://homepages.gac.edu/~arosenth/265/Athenian_Democracy.pdf