Some say classic cars are considered as cars when they are 25 years of age. This is being over-generalised. Classic cars can be further categoried as special cars due to its rarity/limited production, special/unique design, legendary performance/technilogy (at that time) or being regarded a cult icon.
I do agree MX5 would potential be a future classic, but the number of these cars in the world is still very big, therefore, the MX5 has yet to reach a true classic car yet....time will tell.