This question comes up from time to time.
If you know of somebody that can tell the age of every model of every RV brand by looking at it, let me know. The "10 year rule" is there so that a CG owner can say "no" to an RV that is so beat up it may never move again. And folks on this forum and on park reviews will mention it. Not good for business.
Kinda like a CG turning down money if its too wrinkled.
Airstream trailers are classic, and you'd have to be an expert to tell the age by looking, but no CG owner will turn one away. My 2004 'C' turns 10 this year, but when not on the road, its been stored in a heated building since I bought it and looks new. Next year it will magically become a 2005.