Less than one year old. I tend to toss them if the leg elastic starts to give. I hate being at work and having my scrotum's skin pop out the sides. It gets pinched.
I tend to have a lot of underwear at any given time. When I was growing up, the money was spent on my brother and sister. I remember being 17 and still having only the three pairs of briefs I got at age 14. That was all the underwear I had. You talk about pinch. When I got my own money, I started buying lots of underwear. I bet I have at least 20 pairs still in the package.