There are varying theories on this, but my belief is that support and resistance levels are useful because so many traders look at them to determine where to place their stop and limit orders. This means there can be a concentration of buy orders around a support level which could stop the market from trading lower. As long a support level hasn't been broken, it will tend to attract a concentration of buy orders, even if it is an old level, perhaps especially because it's an old level that hasn't been broken in years.