The older models of Aston Martins are more valuable than newer models, and this is because that have a sort of antique value to them. They're worth more because they are older and more rare and, now, more sought after. Limited supply always makes things much more expensive if there's demand.
There are some models of Aston Martins that are definately more expensive than new models. I think that some of them, depending on the model, are more collectible as well. While there are a lot of expensive new models, there are also a lot that are not worth as much as classics.
Vintage cars, regardless if they are Aston Martins or not, are almost always worth more than the newer version. People can buy newer versions of cars easily but older versions are harder to find, especially ones in good condition. The harder something is to find the more valuable it becomes in the future.
Yes, older models of Aston Martins are more valuable than newer models, because they are collectors items. Aston Martins are a piece of history. Each one is made with such care. Really, the people who collect the really old ones are few and far between, but since there are so few left, they are very high priced.
Classic models of Aston Martins fetch huge prices from collectors who have no aspirations for driving the vehicles whatsoever. Aston Martin autos from the 1950s and 1960s are classic cars that have seen a resurgence in value since the Baby Boomer generation has started to retire. Plus, there is something about a classic car that is just a beautiful thing to car collectors.