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Yes, thumb sucking can be successfully treated, but only if she wants to stop. This is also assuming she is not mentally retarded or developmentally delayed in any way. The fact that she is still sucking her thumb at age 25 makes me wonder if her emotional growth isn't stunted in some manner. Problems like thumb sucking can be treated with behavioral therapy. For example, some bad tasting solution can be applied to the thumb which would serve as an unpleasant reminder if she puts her thumb near her mouth. If she sucks her thumb in public, social skills training may also be appropriate.
Best of luck,
Dr. Williams
Monnica T. Williams, Ph.D., Clinical Psychology
No Cost OCD Treatment: http://www.ocdproject.org