Significantly older speakers may still sound good (a speaker that sounds good will always sound good assuming it still performs in good condition). However technology (materials and understanding of speakers) has increased a great deal in the past decades, and the ability to measure speaker performance has advanced a great deal. The very high end, most expensive speaker from decades past today might be decidedly mid-fi, and very affordable speakers today can often keep pace or best some of the best speakers from years past. That being said, there are plenty of great sounding older speakers. But unless there is a particular reason to be looking that far into the past for vintage gear, you'll probably still get better value buying new or newer speakers from a much more recent vintage. There are many things that are measured and understood and designed for today that weren't even known about decades ago, along with significantly more advanced materials capabilities that give modern speakers especially at more budget pricepoints, significantly greater value today than they would have in the past.
But as always, listen, and listen widely, and find the speaker that sounds best to YOU. Whether it be old or new is irrelevant, as long as it sounds good and good to you.