What can actually age
Sheng pu-erh above all, plus shou pu-erh, pressed white tea, Liu Bao, Fu Zhuan and traditionally roasted oolong. What they share is material with enough to give and processing that leaves residual activity — sun-dried leaf can age, oven-dried leaf is stable but inert. Everything else fades: green tea and light oolong lose aroma within a year or two while keeping astringency, which reads as flat and slightly harsh.
Start cheap and recent
Shou pu-erh with a few years on it, or an aged Shou Mei white cake, are both inexpensive and both show what the process does. Lao Cha Tou is almost impossible to over-brew and is the most forgiving introduction to dark tea there is. None of these requires trusting an age claim, because the character does not depend on the tea being old.
Then taste the same tea at two ages
This is the comparison that makes the category legible: Bai Mudan beside an aged white cake, or a young sheng beside one with a decade on it. Everything else held constant, the difference is the ageing. It is also the point at which most people decide whether the direction appeals — it is a genuine change of character rather than an improvement.
The claims you cannot check, stated plainly
A pressing date on a wrapper is a claim, not a proof. Colour deepens with age and can also be produced by warm storage in months. Old-tree material, famous factory marks and storage provenance are all priced and none is verifiable by a buyer. Age is not quality: a mediocre cake kept twenty years is a twenty-year-old mediocre cake, and badly stored tea is spoiled rather than matured.
Storage is your half of it
If you keep tea to age it, you are doing half the production. Stable, ventilated, dry, away from odours, and not sealed airtight — that last one matters, because sealing stalls exactly the change being waited for. The drier and more humid storage schools produce recognisably different results from the same cake and neither regards the other as correct.