How Old Does It Have To Be?

It has nothing to do
with age. It's perception.

When I was young classics could be just a few years old and anything before WW II was almost ancient. Now days some people consider the exact same coins to be moderns. I believe historically people have tended to only collect coins from before they began collecting or from before their own birth. This meant perhaps about twenty years old on average though of course this varied from collector to collector and over time.

Now days a 45 year old coin is still considered modern. This is changing rapidly though as a new generation of collectors is coming who see these coins as old or classic.

1965 is a very good date to make a dividing line since so many things and so much behavior changed then. Later coins are called "moderns" now but the terminology will evolve over time. More importantly though is that perception will change. The idea that moderns are junk will simply fade away over time.

for most series the change from silver to clad is the primary distinction, but in the nickel searchers section we generally use 1960 as the cutoff date since there were no low mintage issues after 1959. For cents a collector might consider 1959, the year when the memorial design was introduced.

I think if either side of the coin is still on coinage you can find in your pocket, its modern. No matter what the other side looks like. As long as there is a Lincoln Cent, they will be considered Modern to me, any cent before Lincoln is classic because its a classic design, nothing classic about Lincoln on a one cent coin no matter what the other side looks like, its still a Lincoln Cent. Same with Nickels, the Buffalo is classic, the Jefferson is modern no matter if the reverse ever changes or not, it will always be a modern design, doesn't even matter what profile they use of Jefferson, if he's on the coin its a Jefferson nickel

Like lots of other people have already said, its
up to the collector him/herself. I consider all lincoln cents before 1941 to
be classic since I hardly ever find them roll searching.

The current split:

Modern US Coins
Lincoln Cents, Jefferson nickels, Roosevelt dimes, Washington quarters, Franklin halves, Kennedy halves, Presidential dollars, Sacagawea dollars, SBA Dollars, Eisenhower dollars, All Bullion Eagles

Classic US Coins
Half Cents, Large Cents, Flying Eagle cents, Indian cents, Two Cents, Three Cents, Half Dimes, Shield nickels, Liberty V nickels, Buffalo nickels, Mercury dimes, Twenty Cents, Standing Liberty quarters, Walking Liberty half dollars, Trade dollars, Morgan dollars, Peace dollars, Flowing Hair Coinage, Bust Coins, Seated Liberty coinage, Barber coinage, Gold Coins

I remember seeing somewhere that the dividing line for the LWC was 1930 and earlier (I think it has something to do with value).