Generally I consider US moderns to be 1965-1999 and anything minted after that to be ultra-moderns. There is some wiggle room for both categories depending on the series. It's a good question and one I was thinking about the other day at the Whitman show. I noticed when I was talking to primarily ancients/medieval world dealers, when I would say that I collect Hungarian Franz Joseph I types (1848-1916) they all referred to them as "moderns". It's an interesting perspective. I consider what I collect to be the classic period of Hungarian coinage, with Hungarian moderns not starting until 1946 with the re-adoption of the Forint. I could see someone saying that Hungarian moderns started as early 1926 with the coins minted under the Horthy regency (I consider those late classic/transitional coins), but clearly anything prior to that is not modern. Perhaps my view on there being a classic period comes from being a US collector though. I wonder if world collectors typically just see ancient/medieval/modern or if this is atypical.
50 years or less. I like the modern silver dollar commems, because I acquired one bu white house SD and 3 above melt for the T.J. Proof SD. You can find a deal if you look. Boy im tired.
If I can go to pretty much any bank and buy examples by the roll at face then it is a modern coin. Has nothing to do IMO with any particular date or the metal used.
Pre-1909 is classic, 1909 (start of Lincoln Cents) to 1986 (the first Silver and Gold Eagles) is modern, anything after 1986 is contemporary.