I completely agree with the article that fresh green tea has more caffeine.
I usually drink green tea that I get from the grocery store. Last week, my coworker gave me some Chinese green tea that she brought from China. The tea she gave me is amazing. It's clearly very fresh because it has a more vibrant green color and a stronger scent. I drank it after dinner and the caffeine in it kept me up! This never happens with my regular green tea!