I think at this point, Ocarina of Time is considered a classic. For me, being a gamer that started playing games far before the 3d standard was put in place, it is hard for me to consider anything that is NOT 2d... classic. Basically, the term classic is very arbitrary and will vary from one person to another.
Does a game have to lose 'modern' playability in order to be considered classic? Meaning, if a games mechanics are BETTER than what some games offer today, can that still be considered classic? To me, it would be still considered modern. If ONLY the graphics were 'slightly' upgraded, and Ocarina of Time was released right now in todays world, (meaning it never was released once before), then I think it would surely be one of the better titles to release for the Wii.
What I'm getting at, is that if a game is still up to par with modern play, than it's not considered classic...... However, I don't agree with this argument that I just made. I think playing Ocarina of Time nowadays is just a tad awkward. I don't think the controls are perfected like current days.
All in all, I don't think pure quality or age has to do with being a classic. It has... an arbitrary aspect to the game that makes it classic. For me, Ocarina of Time is a classic.