Ditto what Minnewaska said about the sails. And you can still sail with baggy sails, just not as fast or efficiently.
For the lines, it is even easier to tell if they need replacement. Are they frayed or worn,
especially in high stress areas such as where they attach to the sails, run through blocks or are routinely cleated off? If so, replace them. Otherwise, they're generally good to go. Also, make sure the ends of the lines aren't fraying or unravelling. If they are, you can whip the ends (wrap thread around the ends), or if the line is synthetic, melt it into a nice glob that will hold together.