American Opportunity Tax Credit

Hi! 

Yesterday I went to file my taxes and took in my W-2 and my 1098-T. When I told the guy to put my form in for my 1098-T he said I wasn’t allowed to receive the credit because the new “tax laws” say we can only claim the credit once every 4 years. I am independent, 21 years old, and this is my 3rd year in college as an undergrad. He said on the tax form when he checked the box that indicated if I was “under 24 years old” that the credit was automatically removed. Is there a reason why I am having this problem if in for the 2017 and 2018 taxes I never had a problem and received my credit ?