When you be a PIRATE!! :D

'Aye' is not an antiquated expression. In most places in England it is less common then 'yes' but in Scotland many people will say 'aye', sometimes more frequently than 'yes' The meaning is exactly the same but as Peachy and Randy say, in America it sounds like something a pirate would say, so perhaps keep to saying 'yes' unless you plan on visiting Scotland:)

Meaning "yes", it is archaic, but still used in some situations. In formal parliamentary voting, the ayes and nays are counted. Sailors still say "aye aye" to indicate compliance. Otherwise, wait until the annual "Speak Like a Pirate" Day.

never

Informally, if in doubt use 'yes'.