Classes in my opinion are determined by class of renters, amenities afforded, and location.
I don't think age of property should be your first criteria. As someone already mentioned, you could have an older building that has now been rehabbed to be a luxury style apartment.
Class A apartments demand the highest rent for the market so that eliminates your section 8 tenants. The government is not putting them up in a room with their own jacuzzi tub.
Class A apartments are normally located in area of city closest to best restaurants, shops, etc. Inside of apartments expect granite countertops, hardwood floors, new appliances.
Class B are properties are still a great place to live and is where majority of your working class tenants are looking to live. They are slightly older in terms of remodel and amenities they provide.
Class B are located in low to mid crime area.
Class C apartments if you are using A-C scale would be your lowest class you would consider (there are D class and F class but for the sake of this thread I'm assuming no one is interested in that headache.)
These class apartments are normally located in medium to high crime areas. Lots of deferred maintenance.
Don't write off these properties. They can have good cash flow and plenty of room for value add opportunities. Just be sure you have a property management company that specializes in managing these type of assets.
Hope this helps @Peter Martin! I am working on an article describing this more in detail and the opportunities each class affords. Ill be sure to post in thread when finished. Cheers!
Last thought as a quick reference in your search
Class A- place where you hope to live
Class B- place where you would be ok living
Class C- Only living there if times were really rough
Class D- hell no, I'm sleeping in my car
Class F- i might as well be dead.