You must pay attention to both quality and age.
Matcha comes in three main grades: koicha (for thick tea), usucha (for thin tea), and culinary matcha. For all three you should use the same criteria you would for selecting vegetables. Look for freshness – vivid, vibrant green is better than dull and dark – and check dates of harvesting and grinding as the older your matcha, the weaker its beneficial effects.
To promote longevity, store your matcha in a cool place in a vacuum-sealed container, something like a mason jar is perfect. This helps stop it from oxidizing and absorbing ambient aromas.