If I understand your question correctly, I can give you a straight answer from working in the business.
Bicycles only depreciate in value.
This industry is much different from others in that 'old' doesn't mean 'desirable'. Old just means junk. Unfortunately, bicycles are not museum pieces either. The general range is about 15 years, beyond that, a bicycle may be good to melt down and sell the metal. Look for the word "steel" somewhere on the frame. There might be a phrase like "high-tensile steel tubing" on it. If the frame says "steel" anything, then it's from the early 80s at the latest. Road bikes stopped being made of steel a long long time ago.
Another easy way to tell the age of a road bike is by weighing it. Lift it up onto it's rear wheel and put the entire thing on a scale. If it's over 25 lbs, it's old. The heavier, the older. This is only for road bikes, other bikes will naturally weigh more.
If that's the case, most bike shops in big cities will turn you away if you bring it in for repair, because the industry is always growing, and getting components for a bike more than 15 years old becomes very difficult. The reason is, manufacturers may go under, or simply stop making those parts. If it is old, and it needs alot of work, a shop may also suggest just buying a new one, because in some cases believe it or not, the cost of getting a very old bike to working like new, may be greater than just getting a new one.
Of course you can still enjoy it and ride it often if it suits you, just be aware that as time goes by, maintenance will become more costly, and may become impossible to find a store in your area that can do the necessary work.