To become epic with a specific class the character must have 21 levels for a base class, or 11 levels for a prestige class. However, the 11th level of a prestige class cannot be taken unless the character has a total of 20 levels already.
e.g. A character with 10 rogue, 10 barbarian and 1 assassin level has no epic classes, but a character with 21 rogue and 1 assassin level is an epic rogue.
Harper scouts and purple dragon knights are only able to attain five levels and may never become epic in the Harper scout or purple dragon knight class.