Each qualifying child you claim on your tax return will be eligible. If your child turns age 17 during the year he/she will not be eligible for the credit for that tax year. Please review the following Turbo Tax FAQ, "What is the Child Tax Credit":
You may be asking about the earned income credit. You can only get EIC for three children even if you have more.
When you enter your dependents and your income into your tax return, TT automatically calculates the child tax credit, additional child tax credit, and includes EIC if you qualify for it. So if you qualified for EIC it is already showing in your refund. If you do not understand the amount of EIC you are receiving, please look at the chart for EIC—look for the number of children you claimed and your income, and you will see how much EIC you are supposed to be getting.
There is a table that starts on page 30 of IRS publication 596 regarding EIC and the amount you can receive for your earned income:
https://www.irs.gov/pub/irs-pdf/p596.pdf
The amount of EIC you are receiving can be seen on your tax return:
1040EZ on line 8a
on 1040A line 42a
on 1040 line 66a
The IRS offers a tool to help you estimate your EIC:
https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit/use-the-eitc-assistant
https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit