While I agree that sometimes the word "classic" is overused - like the word "legendary" - if you have a cut off year, you miss some gems.
For example, a few weeks ago TCM showed TOOTSIE and SOME LIKE IT HOT back to back. TOOTSIE came out in 1981 yet I think it's a classic with a fine, timeless performance by Dustin Hoffman that stands alongside the performances of Marilyn Monroe, Tony Curtis, Jack Lemmon and Joe E. Brown in SLIH.
One thing - were I programming TCM, I'd keep the newer ones to a minimum (yes, even dear TOOTSIE) if they can be seen readily on other channels.