I get this question asked a a lot, and the best place to go is the source!
From the ATF website (https://www.atf.gov/firearms/curios-relics
)Curios & RelicsFirearms automatically attain C&R status when they are 50 years old. Any firearm that is at least 50 years old, and in its original configuration, would qualify as a C&R firearm. It is not necessary for such firearms to be listed in ATF's C&R list. Therefore, ATF does not generally list firearms in the C&R publication by virtue of their age.
A regulation implementing federal firearms laws, 27 CFR § 478.11, defines curio or relic (C&R) firearms as those which are of special interest to collectors by reason of some quality other than is associated with firearms intended for sporting use or as offensive or defensive weapons.
To be recognized as C&R items, firearms must fall within one of the following categories:
Firearms which were manufactured at least 50 years prior to the current date, but not including replicas of such firearms;
Firearms which are certified by the curator of a municipal, state, or federal museum which exhibits firearms to be curios or relics of museum interest; and
Any other firearms which derive a substantial part of their monetary value from the fact that they are novel, rare, bizarre, or because of their association with some historical figure, period, or event.
Use these as your guide:
Curios or Relics List — January 1972 through April 2018 (https://www.atf.gov/file/128116/download
FIREARMS CURIOS OR RELICS LIST (1972 - 2007) (https://www.atf.gov/file/2026/download