Fading and developing are not the same thing
Green tea and light oolong lose aroma and sweetness within a year or two while keeping their astringency, which reads as flat and slightly harsh at once. That is decline. Sheng pu-erh, pressed white tea, Liu Bao and traditionally roasted oolong change in a different direction: bitterness and astringency recede while wood, dried fruit, camphor and earth develop. Only the second is ageing in the sense the word is used for.
What makes a tea capable of it
Two things. Material with enough to give — young sheng is aggressively bitter precisely because it has the substance that decades will convert. And processing that leaves residual activity: sun-dried maocha can age, oven-dried leaf is stable but inert and will not develop however long it is kept. Compression helps by reducing exposed surface so change is slow and even.
Storage is half the outcome
For sheng pu-erh, most of what determines the finished tea happens after it is sold. Drier Hong Kong-style and more humid Guangdong-style storage produce recognisably different results from the same cake, and neither camp regards the other as correct. What is not contested is that badly stored tea is spoiled rather than matured, and that ageing tea wants airflow — sealing it airtight stalls exactly what it is being kept for.
Age is not quality, and the claim is usually unverifiable
A mediocre cake stored for twenty years is a twenty-year-old mediocre cake. Colour deepens with age and can also be produced by warm storage in months. A pressing date on a wrapper is a claim, not a proof, and a buyer has no way to check it. Ageing rewards good material and good storage; it substitutes for neither, and the market’s enthusiasm for age as a price driver runs well ahead of what a buyer can confirm.