DD and I are still at it, at 21.5 months, though now that I'm pregnant, my supply has dwindled to virtually nil. DD does still ask for it, but less often, and doesn't nurse for long. I wouldn't be surprised if she self-weans sometime in the next couple months. If she keeps on, that's fine, too. I'm the only one in our group still nursing at this age, but no one bats an eye about it (that's California for you, I guess... I'm blessed).
The WHO recommends nursing at least to age 2. So that's something to think about.