Ever since pu-erh market sky rocketed, it generates a lot of interest in aged tea. Aging tea is not a new invention and not limited to pu-erh. Aging tea is incidental for most, pu-erh and liu An are exceptions. New crops of green pu-erh and liu an are too raw for digestion.
Why age teas while fresh is usually better? Well, supply and demand are rarely equal. Despite the demand for tea, particular quality teas has increased significantly, there are always unsold teas in any type. What do you do with them? Not much other than keep them around and let it age. China tea industry was at a historic low before mid 80's of last century. Hence you see more aged teas from that period of time. However they were gobbled up by the new middle class of new China in the last 10 years, pu-erh market shows how crazy this phenomenon has been.
Tea stored for a long period of time are subject to moisture, mold, oxidation and pest depending on the condition. How to prevent tea from becoming rotten? Clean environment is a must, next is to keep moisture as low as possible and out of the container. Roasting teas at low temperature will achieve the purpose, then stored in sealed container.
Oolong, green and black teas are all ageble. What kind of tea is suitable for aging? Basically any tea. The fundamental requirement is keeping leaf moisture below 5%. Roasting is the key to preserve quality. Environmental influence is one of the elements to consider during roasting on top of the type of tea to roast, the resulting flavor you would like to achieve. Temperature and duration of roastings are key factors. Younger the tea leaves, lower temperature and shorter time. More matured leaves (more fiber) can handle higher temperature and longer roasting time. There is no formula for roasting, not even the same tea roasted in different days at same temperature and timing will end up with same flavors. We can attempt to not mess up a batch then let it evolve over time. That's why tea is so fun and never boring.