I would not put a limit on how old the speakers are, as that, in itself, does not tell you the quality of it. This is coming from someone who believes that newer speakers are usually better than very old ones, and that the old ones, when new, tended to be far more expensive for any given level of quality (one needs to remember not simply the original price, but also inflation, as $1000 today buys far less generally than it did decades ago).
If you are looking at used speakers, the condition and the specific price you can find will matter very much.
If you are talking about new, but old designs, again, it is the price for the quality that matters, not when it was designed. People have made great sounding speakers for many years, so I would not worry about when a model was designed at all.