Apparently they eventually expire. I got a letter (and an email) on mine last week saying that my transponder has expired and to bring it into a Jewel to exchange for a new one. I can't remember if I've replaced it before. If not, it's from like 98-99. I didn't ever have the first batch that had a display, but I was pretty soon after they switched off of those.
EDIT: Just got the new one, it's good to 2029, so maybe they're 10 years or so? so maybe I got one in 2009-10.
I've had to replace my original one (no display). I just assume there is some form of battery inside and the design does not allow the user to replace it.