If they are already telling you when they went, it's time to start! Jump on this right now before they lose interest... it could be another year or two before they are interested again and might be alot harder then. My 19 month old is practically training herself. She started telling us when her diaper was full around 15 months. About a month ago, I dug out my son's old potty chair and put it in the bathroom. Everytime I use the bathroom, she'll walk in and sit down on the step stool next to the sink... she's been doing that for 3 months... so now I take off her diaper and she sits on the potty. Whenever I notice her diaper has been dry for awhile, I ask if she has to go and she runs to the bathroom tugging at her pants. I sit her down and she pees immediately. Right now we're working on words.. she hasn't said 'pee' or 'poo' yet so we're trying to teach her the sign for potty. No luck yet but yesterday while playing she ran over to the bathroom door and banged on it (it was shut). My son asked her if she had to go potty and she tugged at her pants. So I put her on the potty and she immediately went! Yay!
So completely opposite of the year long struggle with my son. He was interested at 20 months but we put it off due to a move. He lost interested and was not a bit interested a month before he turned 3.. when he HAD to be potty trained to continue in his preschool. So we had a horrible year long power struggle. So now I know what NOT to do... like NOT putting it off and jumping on it as soon as they show interest!