Why is my second round stimulus amount $600 short? Me my wife and 3 kids should be $3000 as the oldest turned 15 in 2020. I was under the impression the cut off was 16 what should I do?

[fanfare]

"what should I do?"

File a tax return to get the amount you find yourself short.

[xmasbaby0]

Actually the "cut off" for child tax credit or the stimulus check is the age of 17.   You lose the CTC in the tax year that a child turns 17.  The stimulus checks have followed that same rule.

You can use the recovery rebate credit for the amount you did not get.

The stimulus check is an advance on a credit you can receive on your 2020 tax return.  If something went wrong or you did not get the stimulus check in 2020, you can get it when you file your 2020 return in early 2021

The Recovery Rebate Credit will be found in the Federal Review section,  If you are eligible it will end up on line 30 of your 2020 Form 1040.

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-...

https://www.irs.gov/coronavirus/coronavirus-tax-relief-and-economic-impact-payments-for-individuals-...

https://ttlc.intuit.com/community/tax-topics/help/how-will-the-stimulus-package-impact-me/00/1393859