Was this from the IRS? Did someone else claim your dependent? You may have to call the IRS and find out what documentation they need. But to answer your question, if you have a qualifying child, then you can claim the credit when you are under 25. If you do not have a qualifying child, then you have to be 25 to qualify for the earned income credit. So it sounds like there was not a dependent on your return. If not, then you can amend and add your dependent.
Go back and double check your return information especially all the answers for your child- including age, Social Security number, and all the related questions about support etc.
