I start to put my hands in my mouth around 14 months old. It has never left me yet! They are very invasive, wiggling in my mouth all day at the exception of my sleeping state.The good news is that I do not hurt myself, my fingers are like soft and gentle tools to me. I wear acrylic mittens all day, it relax me to not have my fingers in my mouth at all time, but with time I am getting used to my mittens and hold them closer and closer, getting them all wet. My mom has a bunch of colorful mittens for me, I probably use eight pair a day. Right now I am growing and changing , so my hands are very very active, can't wait for calmer time.