I'm a girl who lives in jeans. They're my uniform, day in, day out, and fortunately for me that's ok for work too. And the more ancient they are, the more comfortable they are. Old friends really. I've lost track of how many pairs I have, but I have noticed that most of them are worn at the knees and need some attention. Not that I'm against the torn look, but just not on every pair.
My old Bernina has a darning option, which is perfect for sewing over the ripped and worn bits of denim and strengthening them, in a relatively discreet way.
If the denim's really thin I put a small patch on the inside to make the repair a little more substantial.
So...more life in these old jeans yet...and several more pairs to repair too. It helps to pay attention while you're sewing - a momentary minute of mind wandering and I neatly sewed right through my finger nail, with the broken needle coming out of my fingertip. Ouch!