When cars were first fitted with airbags there was a lack of knowledge and confidence about the long term reliabiilty of the systems . Car manufacturers and manufacturers of airbag systems agreed to validate (or test) them for a 10 year life span.
A sodium azide tablet produces the gas which inflates the airbag, and this tablet and the explosive device are sealed. Although some moisture ingress is possible it should not affect performance. There will also be some natural chemical decay, but again this shouldn't stop the airbag from working.
There was also some concern in the early days that the airbag cover could become brittle through aging and a flying fragment cause eye damage.
Manufacturers increased the validation period to 15 years on later cars without making major changes to design and we are not aware of any airbag failures atttributed to ageing.