Although I agree that 6 months might be too young to night wean, I don't think you need to wait until 18 months to night wean. It really depends on your situation, your baby and how much she is getting up at night. IMO, if she gets in the habit of nursing for comfort ALL of the time, it will fragment both her sleep and yours and neither of you will be well rested.
My dd, 12 12 months old was starting to wake up ever 1 1/2 -2 hours to nurse for comfort. We decided to try Elizabeth Pantely's Nighttime approach to reduce the nursings. I began doing this when dd was about 11 months old. I didn't want to wean all together, just to reduce the wakings. Now, we are trying Dr. Jay Gordon's approach, because, again, I want her to get a longer period of consolidated sleep. Basically, she goes to bed at about 7 p.m. and I nurse on demand until about 11:30 p.m. From 11:30 to about 6:00 a.m., I try to comfort her to sleep with other methods...back rubbing, dh holding, singing... We did this method a few weeks back and it was working but then we got side tracked with other things. We are now starting it again. If you are serious about doing some form of night weaning, I would suggest taking a look at the No Cry Sleep Solution Book by Pantely and Dr. Gordon's site at www.drjaygordon.com.
Again, I wouldn't really recommend doing this until dd is a little older In fact, Dr. Gordon doesn't recommend any change until the baby is at least 12 months old. Use your own best judgment where that is concerned. Good Luck!!