Antique dealers generally refer to anything over 100 years as an Antique. So does Canada Customs, for import purposes. So that's the Government of Canada EXCEPT that the Government of Canada ALSO says that firearms are only Antiques if they were made previous to 1898, are single-shots if long arms and use ammunition which is not commercially available. Regulations, also from this very same Government of Canada, are somewhat different if the firearm in point is a handgun. As an example, I have a little Iver Johnson .32 which was our local Police pistol. It is DEFINITELY over 100 years old (it was already old when it performed a celebrated arrest in a local coffee shop, 90 years ago) and the Government of Canada says that I may keep it only until they decide to torch it.
So the "Antique" thing is a can of worms.
Personally, I would include the Great War as "Antique", there no longer being any of the active participants living.
"Classic" is something different, to my way of thinking. To be a "Classic", a firearm should possess some characteristic which makes it stand out from the run of the mill. An SMLE is a "Classic" if ever there were one, but the much scarcer Conversions are not. The Number 4 today is becoming recognised as a "Classic" although it was not, only a few years ago. The FAL is a Classic, even if we aren't allowed to take them out of the house in a so-called Free Country of Free Citizens.... but the BM-59, just as efficient a rifle, is not. Again, a whole can of worms.
It is all just so VERY confusing to a small brain such as mine.