From that article a few of the key points to deem classic were.
Iconic
Sought after
Character
For me the 350 ticks all those boxes
Iconic - Yeah, the return of the Z cars, featured in many 'popular' skid films. Was the focus of one of the biggest Playstation games and spawned the birth of the GT Academy. Plus a profile that from whatever angle you see one, you just know what it is.
Sought After - People are still buying them, value seems to be holding quite well. I'd bet a lot could be sold for what you paid after 3 years
Character - As per the article, all of the 350's on here have a story and their owners have that special bond. How often do you park up at the shops and as you walk away you just look back and smile ? Even those who ventured to other cars still seem to get the bug and come back for more. Not to mention all the little 'quirks' the 350 has.
I may be biased, but if we can't deem the Z a classic now, then surely in the next 5 years tops they will be.
As mentioned above, Modern classics can be anything over 15yrs old, that obviously covers the early DE's so can only be a matter of time....