In either 1998 or 1999 the United States mandated driver and passenger airbags in passenger cars, mandating the same in pick-up trucks by the end of 1999. Basically so people don't get hurt as bad hitting other solid objects

Usually they say it's impossible to predict the reliability of a replacement or modified airbag,
especially if it isn't a certified one made by the company who also created your vehicle.

I'm sketchy on the idea about the last owner installing an airbag in a vehicle that originally didn't have one. It makes as much sense as buying a base model car without ABS and then buying the ABS system from higher-end models and putting it in your vehicle.
According to my Gen4 Camry, you have to get the airbags inspected ten years after the date in which it was made, and then once every two years after that.
You should read up about how airbags work on HowStuffWorks.com in order to understand everything about them. It's quite neat