Lifespan of backup tapes

Currently I am using IBM LTO Ultrium 100Gb data cartridges for my backups. And I have been trying to find out the lifespan of each of this tapes before it give ways. And this is really worrying, as the oldest tape use is from 2002.My question is

1)How do i determine the ageing of this tapes?
2)When should the tapes be change?

Any answer is very mush appreciate.

Rgds.