Old aircraft are generally categorized by the technological development of military aircraft that in turn influenced the progress of general and commercial aviation. Thus, “old” or vintage aircraft tend to be typed as either pre-WWI, WWI, WWII, and post-WWII. Not many pre-WWI and WWI aircraft are still flying, mostly because they are extremely difficult to maintain as airworthy. But WWII aircraft remain in rather significant numbers and in surprisingly good condition.
Aircraft design is the epitome of the principle of form follows function. An aircraft must be designed and engineered first and foremost to function as an efficient and safe flying machine. Hence, their form is purely the result of their function. Eye appeal is secondary, perhaps not even relevant. But, as of the result of the most critical consideration to function, aircraft tend to be beautiful in their own right.