I have been told and agree that the best time is when they start being aware of when they are filling their nappies, when they start telling you. My son is 2, he tells me when he has gone, but hasnt got the knack for informing me beforehand, Ive asked him about the potty and he seems reluctant, he'll sit on it when clothed but not naked. Im giving him a few months then will ask him again, I dont want to push it because if I make a scene it'll make it harder in the longrun! So I say go with the flow (no pun intended!) when shes ready she'll tell you.
I have heard stories of some people having their child potty trained by 18 months and if that has worked then thats great, it really does depend on the child.

i started my oldest a few months shy of 2 yrs but she only went in the morning and before bed. We only did that for a while. When we really started (a little after 2), we used the sticker system and she got the peeing down quick and was in chonies before any really fit her. She used pull ups at nap time for a few more weeks, but just recently was able to get out of them at night (5 soon). My son I started at about 2 1/2. We used hugs for rewards, and he ran around naked (tile flooring) and it took all of 2 days to get him to go potty in the bathroom. Again pull ups at nap and night, but for the last few months (since about 3 1/2) he has been out of them all the time.

My pediatrician recommends not starting before 2 as they arent REALLY aware of what they are doing, most of the time. I happen to agree with her, and also that its best to wait until they are ready. You can try when you want them to go, but dont force it as it will make it take longer.

Ive been wondering the same thing. My daughter will say she pooped or peed but she didnt. She will be 2 next week. But at daycare there is a child who is being potty trained and my daughter always wants to sit on the potty. I dont think she really understands yet. Im hoping in another few months she will be ready...