I believe it is the gas in the triggerring cannisters that becomes weak. The bag will deploy, but with not enough acceleration to protect you from the crash. The nitrogen gas becomes more inert over time.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)