Anything 1959 and newer, but will admit the date 64 is to me the dividing line of coinage I collect and coinage that is just IMHO made for commerce . There's very little after 64 that tweaks my interest .
Could be any or all of the above. For me it's a bit different. Modern commemoratives started in 1982, modern dollars started in 1971, both after a long hiatus of not being made. For those in Europe it might have started with the new Euro coins and in Canada it might have started with the close of coinage that went from paper to Loonies & Twonies. For Germany it might have been with the close of the Nazi era coins & paper. A bit different for everyone I suppose.
It depends on the series, but if I'm still spending it, it is modern. Even though Lincoln has been a cent for over a century, it is still a modern to me.
I put it at around 1947 , when the last beautiful coins were made , ie Mercs and Walkers and Standing Liberty quarters . I do like the early Lincolns , but hate the new zincoid low relief junk .
At 1980 I'm pretty much fading out on the cents, nickels, and big dollars. I never liked the small dollars that much. On the dimes, quads, halves, I guess when they took the silver out.
It's weird how the design was the same but they just kept getting worse each year. The first 10 years of the series were the nicest.
I basically lose interest in copper after the size reduction. Silver post seated coinage. I do like slq and walkers tho. But for me modern is really anything that doesn't have a 17 or 18 as the first half of the date