I think that one has to be a little mature to like the classics, just like cinema or music. A 13 year old boy prefers action and colorful anime with a simple plot...
not just anime, the same goes for movies and animation in general (books are out of the question) and this does not go only for 13 year olds
Problem was we were too young to really appreciate the classics back then and now we're too mature to like the new series. A proof that the older series were overall more sophisticated for the age group they targetted. Fortunately the cartoons till the 90s were better and this is not subjective. Sure a kid will find any cartoon great but the cartoons back then had more quality
I dont blame the younger viewers. I was the same in their age. But the younger ones with the Internet are more lucky than we were since they can pick any show they like and enjoy it much more. They just need the right guidance.