That question is more complicated than it may appear on the surface because airsoft guns evolved from [non-firing] model guns, and early airsoft kits were somewhat different than what we know today -- having barely any power or accuracy, however being a model gun they were extremely detailed replicas with top notch aesthetics.
The first airsoft gun is usually attributed to MGC (defunct since the 90s); Hudson Model Guns is an old model gun company (which is still operating), which was among the first to produce domestic model guns along with MGC, but while they eventually did make airsoft models I think their first airsoft gun was released well after many other companies' -- so they may be the oldest company still producing airsoft, but other companies may have been producing airsoft for longer. Hudson, which is often considered lower quality for our needs, is a good example of "early" airsoft guns (model guns that often aren't really designed to be gaming workhorses). When MGC closed up shop their tooling was bought up by KSC.
Maruzen and Murushin are two other early model/airsoft gun makers that are still around; other companies like JAC, Asahi and Kokusai (all defunct) are considered "classic airsoft." Their, along with MGC's, demise came around the same time Tokyo Marui (a R/C toy and model maker) put out an electric -powered airsoft gun in the mid 90s. Having a self-contained rifle rather than having to haul an external gas tank was considered much more realistic and attractive by most and AEGs quickly gained in popularity and helped establish airsoft as the sport we know it today.
Last but not least is Western Arms, originally an aftermarket parts maker for MGC 1911s and Beretta 92s, but they introduced several innovations including something called Magna Blowback: MGC and JAC had been the first companies to introduce blowback guns but WA's Magna system was by far superior and its general design and operating principals are now used in all blowback pistols and some rifles (such as WE's open bolt system and many SMGs).