Remember that when Mercedes (or anyone else for that matter) assigns a "life" to something like an airbag it is usually assuming the worst as to conditions. The last thing any of these manufacturers wants is to get sued by someone with a 20 year old airbag because "you never told me I had to replace it." I'd guess that on average these bags are generally good for far more than 10-15 years, especially if the car is stored out of the sun or whatever makes airbags go bad faster. The 10-15 years probably represents the minimum time before the first few airbags from a production run might begin to go bad. If you're willing to take a bit of a risk I'd guess they're mostly still good after even 25 years. You have to decide if the slight risk of it not deploying vs. not spending the $$ to replace it is worth it.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold