Yes, green tea, and most other teas, lose quality in storage. I recommend enjoying your green tea within 12 months of the harvest date for optimal quality. Other factors that will degrade the quality of your tea during storage are excessive heat, frequent fluctuations in temperature, direct light, and exposure to air.
To make sure your tea lasts as long as reasonably possible, use an air-tight container that protects from direct light. Also, be sure to store your tea container in a cool dry area.
One tea that does not lose its flavor, but actually improves with time, is called Pu-erh.
Pu'er or pu-erh is a variety of fermented tea produced in Yunnan province, China. The town of Pu'er is named after the tea that is produced close by. The fermentation in Pu’er tea production involves microbial fermentation and oxidation of the tea leaves after they have been dried and rolled. It is this fermentaiton that allows pu'erh to improve with age.
Tea enthusiast and owner of Tao Te Tea