Guardian of the Ages — ruling 1

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.

Guardian of the Ages — ruling 2

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).

Guardian of the Ages : règles, fonctionnement et interactions (MTG)

Guardian of the Ages is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Defender (This creature can't attack.) When a creature attacks you or a planeswalker you control, if this creature has defender, it loses defender and gains trample.

How it works : Guardian of the Ages

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2013· Wizards of the Coast
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.
No other card is named in this ruling.
#2July 1, 2013· Wizards of the Coast
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).
No other card is named in this ruling.