She is a qualifying person for the credit if she is:
A person who is physically or mentally incapable of self-care, lived with you for more than half the year and either: (i) is your dependent; or (ii) could have been your dependent except that he or she is over the gross income limit or files a joint return, or you (or your spouse, if filing jointly) could have been claimed on another taxpayer’s 2014 return.
An individual is physically or mentally incapable of self-care if, as a result of a physical or mental defect, the individual is incapable of caring for his or her hygiene or nutritional needs, or requires the full-time attention of another person for the individual's own safety or the safety of others.
See the IRS Tax Topic 602 for more details on the credit.
Nowhere on the 2014 Form 2441, which is used for the credit, does it ask for the age of the dependent. Nor does it ask for that on line 6 of Form 1040, which is the line where you put your information about dependents. The IRS will match the information with SSA, though, and will know the age of the dependents you claim. The IRS won’t “automatically deny” the credit. It might send you a letter at some point asking you to explain how the dependent qualifies for the credit, however. If you qualify for the credit, go ahead and take it.