My son is 2 weeks old and he sucks his thumb since day 1:/ not sure how I feel about this. I'd rather him take the Paci instead but he wont!
I had to train my first early, newborn age, and pull fingers out of mouth. I think it took a while but eventually she got it. That was one habit I didn't want her to have. I had a friend who sucked her thumb well into high school!