I reinforce jeans with a layer of fabric on the inside, and zigzag stitch around the edges. Machine darning, basically just sewing lines across the worn areas and into less worn fabric nearby, can help hold things together for a little while longer. Or you can do the same thing by hand, like the Japanese sashiko mending/quilting technique.
Check out some of the YouTube videos on visible mending too - that's having quite a moment in the sewing and textile world.