What a philosophical question!
I had to write an article about this at school, but I still find it very difficult to form an opinion.
A classic for me, should be something that lives on, something that just can't be ignored. So new books can also be instant classics (Although I think people throw this term around way too much)
But I am aware that most people don't like old books, or even books in general. So classics just become important to a small group of people that are interested. Which might show that all taste is personal and talking about it can feel pretty pointless.