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.
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).
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.