I think it is very difficult to generalise about design / build construction based on age. There are some very good designs available from every decade, unfortunately there are also crap designs / construction in every decade as well. Even with the same design I would prefer a boat that was laid up continuously (and so the resins in each layer bonded correctly) rather than a yacht that was half finished over a long weekend. However this would be bloody difficult to determine.
When I purchased my current boat all the one's I was looking at were 25-30 years old and some of them were asking 20% less than other boats. One thing I used was my nose, ie most of the cheaper boats had a musty smell about them that warned me that water had intruded into the wrong areas. You could spend thousands of $ fixing these issues, plus it is an indication that the sellers probably let other maintenance issues be forgotten.
JC unfortunately I do not believe there is much you can do to determine whether a boat was well constructed, or poorly constructed 20 / 30 / 40 years ago, apart from researching the reputation of the builder and seeking comments from a knowledgable surveyor. Once you get past 20 years prior maintenance is much more of an issue, ie a well designed / constructed bulkhead will be useless if water intrusion has rotted out the wood.