My AGM is past six years, seven years in spring.
The apparent advantage of AGM is due to the coincident improvements in the batteries’ environment. The enemies of battery life are heat, dirt and vibration.
An actual advantage is their fragility to too high charging voltage / current, due to their limited electrolyte volume that cannot be replenished. Excess gassing is a function of temperature and voltage. To avoid this fragility requires the AGM be gently used.
AGM battery service life is correlated with the Average State of Charge, with average SoC = 70% implies 1,000 equivalent cycles of full rated discharge and recharge.