The short answer
For oolong the answer depends entirely on the roast, and anyone who tells you otherwise is selling something.
Light oolongs are best fresh. They should be drunk within about 18 months of roasting, and they decline steadily after that.
Heavy roasted oolongs are the opposite. They improve for five to ten years and then hold for a long time after that.
So there is no single answer for the category, which is why this question keeps coming up.
If you were buying one oolong to drink this year, buy a fresh light one or a recently roasted heavy one.
Buying one to keep means a heavy roast, and then leaving it alone.
A medium roast, around 40 to 50 per cent oxidation, sits awkwardly in the middle and does neither job especially well.
Traditional roasted Tieguanyin and aged Wuyi rock tea are the two names that justify keeping oolong.
Everything else in the category is at its best in the first year or two, and it is not worth waiting for.
The short version: green oolong for now, dark oolong for later, and do not confuse the two.
Why age matters differently
The reason is what is left in the leaf to change. A light oolong has volatile aromatics and nothing else working for it.
Those aromatics fade, and there is no process running in the background that replaces them.
A heavy roast has been heated hard enough that much of the volatile material is already gone, and what remains is more stable.
Over years the roast settles, the harshness rounds off, and the tea becomes thicker and darker in the cup.
Moisture matters too. A heavy roast is dried harder and keeps better, while a light one holds more water and spoils sooner.
That is also why a light oolong needs the fridge and a heavy one does not.
Oxidation level is a rough guide: below about 30 per cent, drink it; above about 70 per cent, you can keep it.
The middle ground is where most of the famous names sit, and it is the hardest to call.
Re-roasting complicates it further, because a tea roasted twice runs on a different clock from one roasted once.
None of this makes old automatically better. It makes old different, and different is sometimes worth having.
How to tell
Ask for the roast date rather than the packing date, which is the most useful question in this category.
Most roasting happens in autumn, so a roast date within the last twelve months means a fresh tea.
Look at the leaf. A light oolong that has faded is duller in colour and has lost its sheen.
Smell the dry leaf. Freshness in a light oolong smells floral, and age smells like nothing much at all.
For a heavy roast, smell for depth rather than freshness. Dried fruit and charcoal are good, and flat wood is not.
In the cup, an aged heavy roast should be thicker and darker than a fresh one of the same tea.
If you can, buy the same tea fresh and aged and drink them together. That is the only proof that matters.
Vacuum packing is a strong hint that the seller considers the tea fresh and worth protecting.
A paper-wrapped, loosely packed oolong is usually intended to be kept, because paper lets it breathe.
Price is a weak signal. Aged oolong is often cheaper than people expect, and bad aged oolong is not cheap at all.
Eighteen months is the ceiling for a light oolong, and ten years is a reasonable horizon for a heavy roast.
Storage caveat
Ageing only works if the tea is stored properly, and most home storage is not good enough for a light oolong.
A light oolong wants a sealed bag and a fridge at around 5 degrees, and it wants to stay there until you open it.
Once opened it behaves like any other tea and should be drunk within a few months.
Heavy roasts want a sealed tin, a stable cupboard, and no attention whatsoever.
Heat and light are the enemies in both cases, and a shelf above a kettle is the worst place in the house.
Do not store oolong next to anything with a smell, because it takes on whatever is nearby.
Do not open a tin to check on it. Air is what you are keeping out, and every opening lets some in.
Check stored tea once a year rather than more often. Opening it monthly achieves nothing but exposure.
If a stored oolong smells musty, it went damp at some point, and no further keeping will fix that.
Stored badly, a ten year old oolong is worse than a fresh one, and there is no prize for the years.