I think something becomes "old" when a newer model comes out and the styling changes so that it looks significantly different.
Agreed. My definition would be "it becomes the old one when there's a new one."
That doesn't make it old, just in comparison (as everything in life).
If you have a phone you bought a month ago (say, an iPhone in August), then a new model comes out (in September), if someone saw your phone and asked "Is that the new iPhone?" then answer would be no. Even though it's just a month old (the model itself being a year old), and still "new" to you, it's not what they mean.
This guy was referring to a redesign of the car, and it being the "old" style in comparison. It could become "old" when it's being sold brand new, if a newer model is out (and they're blowing out all the old inventory).
It doesn't mean anything, being old, or new... if you're happy with it, what's it matter if something is "new" or not? :)