All children learn at different rates. While children naturally learn to walk and talk just by being around other people, alphabet recognition, as with all literacy skills, has to be consciously taught.
 
Does your daughter have pre-literacy skills, such as alphabet recognition?  Is she completely unable to read or lacking in fluency or reading but not at grade level?  
 
I am answering as if it is the first.  If she has some literacy skills, please add information in the comments and I will come back and revise my answer to better suit your situation.
 
You do not mention if your child is enrolled in school, if you have made a conscious effort to teach literacy skills, if your child has been identified as having a learning disability, or if any pediatricians or educators have mentioned concerns in the past.
 
Certainly by age 15 I would hope that an adult (an educator or family member) would have made an effort to teach literacy and raised concerns if these efforts were unsuccessful.
 
If she is enrolled in school, by now the school should have evaluated her and developed an appropriate, individualized program.
 
Education.com has some great activities to make learning to read an enjoyable experience: http://www.education.com/topic/reading-and-the-alphabet/
 
At your daughter's age, I would recommend wasting no time in seeking out an evaluation and enrolling her in a literacy program.
 
I urge you to bring your concerns to your pediatrician and to your child's teacher, if she is enrolled in school. These professionals may recommend a specialist who will administer diagnostic tests and work with your child.
 
If you are in the United States, your child is entitled to a free and appropriate public education, including individualized support for diagnosed learning disabilities.
 
More about learning disabilities and Individualized Education Plans here: http://www.education.com/topic/learning-disability-information/
 
I want to emphasize that many children with learning disabilities have average or above average IQs. And many successful people have been diagnosed with learning disabilities. A very inspiring list may be found here: http://www.greatschools.net/LD/managing/famous-people-dyslexia-ld-or-ad-hd.gs?content=696
 
There are a number of reasons why a child may struggle with reading and many of them may be overcome with additional support. A diagnosis of a learning disability does not mean that your child will not eventually read at or above grade level.
 
Reading can open up entire worlds and I wish your daughter a great experience learning to read!
 
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Candace Lindemann, Ed.M. - JustAsk Expert
Educational Consultant and Writer
http://CandaceLindemann.com