Many later problems of children are rooted in inadequacies in this first, trust - forming stage — depression, feelings of unworth, withdrawal from relationships, continued infantile behavior such as thumbsucking and overeating, for example. (religion-online.org)
Thumbsucking in infants is quite common and very natural. (carolinaparent.com)
The ADA reports that children usually stop thumbsucking between ages 2 and 4. (carolinaparent.com)