Problem: The IRS - not eFile.com - rejected your tax return because you checked the box indicating that your dependent qualifies for the Child Tax Credit, but the dependent is 17 or older.
Solution: According to the IRS records, your dependent was 17 or older by the end of the tax year. If that is incorrect, please check the dependent's SSN and date of birth as entered in your eFile.com account to make sure those are entered correctly. To find your dependents, click on Federal Taxes -> Dependents in the left menu, then click on the name you need to check. Make any changes needed, save them, and try to e-file your tax return again.
If your dependent was in fact still 16 or younger by the end of the tax year, then the IRS and/or Social Security Administration must have data wrong in their database. If this is the case, you would need to contact the IRS and Social Security Administration to verify your dependent's SSN and date of birth.
To correct any errors that may be present, you can contact the SSA at 1-800-772-1213 and the IRS at 1-800-829-1040. You may experience long hold times or not being able to get through right away to these two agencies, but do not give up as they are very busy right now. You may have more luck finding the phone numbers for any local SSA or IRS offices that might be in your state.
After any update to their systems, you will probably need to paper file your tax returns.