Guardian of the Ages

Oracle Text

Defender (This creature can't attack.)

When a creature attacks you or a planeswalker you control, if Guardian of the Ages has defender, it loses defender and gains trample.

Card Rulings

7/1/2013 The effect of Guardian of the Ages’s triggered ability doesn’t wear off as the turn ends. Losing defender and gaining trample last indefinitely.
7/1/2013 Once Guardian of the Ages has lost defender, its triggered ability will no longer trigger when a creature attacks you or a planeswalker you control (unless it has somehow gained defender again).