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Frame design and materials is only part of the puzzle. For one material technology has only got better with time, e.g. a modern aluminium frame is significantly better than a late 90s aluminium frame. Also things like internal headsets, tapered head tubes, modern bb standards, etc all add up to a bike with is stiff where needed, more compliant where needed and all those stereotypes. Not to mention the modern parts which fit those modern frames.
Ride quality isn't related to age. It depends on frame design and the materials used.
Serotta excels with both of those.