From my 1869 dated 1866-74, to my Sig 556, and all in between, being mostly a WWI guy, about 80-90 years.
With the exception of one modern rifle and one modern pistol, the youngest gun is a 1942 Longbranch.
My recent foray into mosin territory has brought the average age down... At least until the refurbed 1897 comes home from layaway.