All my old jeans are disintigrating at the same time. Do Levi's have an expiration date?

I had 3 or 4 pairs that I used for gardening. Knees ripped open, crotch frayed. I put on one pair a few weekends ago and did some work and then ran some errands and noticed a nice breeze on my bottom. Turns out the place where the back pocket meets the pants had sprung loose and my butt was flapping in the wind.

Put that pair in the trash and the following weekend put on another old pair. Falling apart in the same place. That's two pairs of garden pants to go into the bin.

What if they're all bad? What will I wear for gardening? I should buy some more Levi's now so I have something for gardening a few years from now.