A tablespoon is a non-scientific but very widely used unit of volume, which works very well in cooking, herbal medicine, etc. It is represented by a capacity of one tablespoon which varies from roughly 15 mL in the US to 20 mL in Australia.
1 If tea is properly stored, it can potentially last up to a year for green and black teas. White tea lasts about six months before it isn't as good in quality as when fresher. Stale tea (or bad tea) will taste flat, as if you're drinking paper.  Glass or ceramic containers are fine provided that they're opaque.
Depending of the variety of green tea it should be brewed around 175 degrees F (79 C) give or take 5 degrees. This can vary on the variety so be sure to check that carefully. You started off with old green tea. Green tea, when properly packaged, has a shelf life of about 6 months. Once opened you have about 2 months to use it. That's for properly packaged (vacuum packed or nitrogen packaged) tea. If your green tea wasn't packaged properly and is exposed to any amount of air, it probably was never good to begin with.