I'm going to get mine replaced when it gets to 20.
Shows good maintanace and meticulous ownership.
Plus, it may work out to protect your Benz in an accident. It looks better to an insurance company in the sense that they may be less likely to write it off if the airbag deploys if you have receipts showing a very recent replacement. Also not sure if an insurance company would play games with any bodily injury claims if they knew an airbag was "expired" - this is new territory. I'd also consider some type of maintanace or replacement on the self tensioning seatbelt system as well while you're at it.
Ask your dealership.