Once tea is properly brewed, the liquid should be separated from the leaves immediately; otherwise extraction continues and the tea gets harsh. All kinds of tea are best drunk fresh; as they stand, their aroma dissipates, and their phenolic components react with dissolved oxygen and each other, changing the color and taste.
Tea is sometimes mixed with milk. When it is, the phenolic compounds immediately bind to the milk proteins, become unavailable to bind to our mouth surfaces and salivary proteins, and the taste becomes much less astringent. It’s best to add hot tea to warm milk, rather than vice versa; that way the milk is heated gradually and to a moderate temperature, so it’s less likely to curdle.