Well, from what I found -- as far back as 6 months old! This is a picture of Charlotte from a couple months ago. She was playing shy girl, but notice the fingers she is sucking on.
Now, this was Caitlyn and Charlotte from our Corpus Christi, TX trip in April 2006. Caitlyn is on the left and Charlotte is on the right, sucking on her fingers. They were a little younger than 6 months old then. I guess she started that habit young!
And, this was a picture I found and I consider it one of my favorites. I love how they are looking at each other. They have their moments now, but they are pretty much inseperable.