There's no point in trying to convince me that you should get a larger credit. It's not up to me, or to TurboTax, to determine the amount of the credit you get on your tax return. The amount is determined by the tax laws. Below I will show you how you can see how the tax law determines your credit.
If someone at the IRS told you that they were including 18-year-olds in the second stimulus payment, either that person didn't know what they were talking about, or they misunderstood your question, or you misunderstood their answer. According to the IRS's own statistics, their phone reps give wrong answers to about 1 out of every 12 questions.
You received money for the 18-year-old in the third stimulus payment because the law that established the third stimulus payment eliminated the age limit for the third payment. But the under 17 limitation still applies to the first and second stimulus payments. You didn't say exactly how old your son is. You might have gotten the first stimulus payment for him based on your 2018 or 2019 tax return if he was not yet 17 in the year that your first payment was based on.
You can see exactly how the Recovery Rebate Credit on your tax return is calculated. In the IRS instructions for Form 1040, look at the Recovery Rebate Credit Worksheet on page 59. This worksheet is the IRS-defined method for calculating the credit. TurboTax uses the same IRS worksheet. You can work through it by hand and see how much credit it gives you. Read the steps on the worksheet very carefully. In particular, note that lines 6 and 9 say "the number of qualifying children under age 17 at the end of 2020."
