For me, I associate a classic with age as I feel a classic is something that is timeless and does not lose it's appeal regardless of how old it is. So yeah, there are plenty of old films that fit into that bill.
I have to be honest and say I don't really have many old films in my collection. The oldest may be some Clint Eastwood Westerns such as High Plains Drifter and the afore-mentioned The Good The Bad And The Ugly. I would also add some WW2 films in the mix, such as Kelly's Heroes (Eastwood again!) and A Bridge Too Far. I have no doubts that the more recent Saving Private Ryan will already be considered a classic and that is only 15 yrs old (I think!) Another modernish film I consider a classic is Blade Runner. I could go on all day, so will shut up, now!
N.D.
Highlander!