Interesting question I have often thought about. I really think it depends. I know a fellow that has a 22 that was bought when he was 12, passed to son when 12 and then grandson. It is 112 yrs old. The weather, care and money all make a difference. We know there was a yankee (Jn Burns) at Gettysburg that was upset the Southern army was invading the north (it was all right to invade the South though) he was using an old cut down musket (probably 1816), so that would be 46 years old at the time. The Mexicans used Paget's carbines in the Alamo and Mexican War (about 40 years old at the time). Jim Beckwourth I believe carried a 3rd model Brown Bess.
But, again, what was the station in life, abuse it took etc. I am 54. I love the 1850-65 period. I am too old to reenact as a guerilla anymore, so I try to portray the station I am (would have been (under the same circumstances) had I lived then. I am NOT wealthy, so I do not portray Tara Plantation. I have a small farm about 60 acres, and I work outside as well. We have enough, but not much extra. My clothes and firearms I use reflect that.
I have a Jackie Brown French style fusil from around 1780. I would have been born in about 1800 to be the age I am in the 1850's. Could I be using a 75 year old piece? May be.... But I wouldn't bring that to a reenactment. I would use a shotgun or 1841. Personally though, at home, I may use it with my 1850/60 clothes and mentally tell myself it was my father's and still works.
I agree that with all the fancy carved wood, those would not have been the rifles used for hard use in the woods. Personally, I would want to stick within 40 years of what you portray. There are historic anomalies, guns that shouldn't be found where they are, guns that were not issued etcetera. But most folks used what was common to the area and time, like we do.
Just my opinion,
David