The short answer
For red tea the short answer is new, with almost no exceptions worth planning around.
Red tea is fully oxidised during production, so there is no fermentation left to run and nothing developing in the bag.
What happens over time is a slow loss of the top note, and then a general flattening.
It keeps better than green or white tea, which is why people assume it improves.
It does not. It declines more slowly, which is a different thing entirely.
A good red tea is at its best in the first year and perfectly drinkable in the second.
By the third year it is flat, and by the fifth it is a different and lesser tea.
There is one partial exception, which is a Chinese red tea stored for a couple of years to round off the edge.
The change there is small and mostly about harshness, and it is not what most people mean by aged tea.
So buy red tea to drink, buy it often, and do not keep it for a better day.
Two cups a day is about 500 grams in six months, which is roughly the most to buy at one time.
Why age matters differently
The reason is simple: red tea has already been through the oxidation that other teas do slowly in storage.
Once that step is finished there is no process left that improves the leaf.
What remains is a slow loss of the aromatic compounds that give the tea its character.
Moisture is low, so it does not spoil in the way a damp tea does. It simply fades.
That is the whole difference between red tea and dark tea, which look similar and behave completely differently.
Dark tea has microbes working in it. Red tea has nothing.
This is also why the market for aged red tea is so thin compared with the market for aged pu-erh.
A few Chinese red teas are stored deliberately for a year or two to soften them.
That practice removes a harsh edge rather than adding anything, and it has a ceiling.
Anything sold as decades old should be treated as a curiosity rather than as a purchase.
Two years is the line: before it the tea is itself, and after it the top note has gone.
How to tell
Ask for the harvest date rather than the packing date, because the two can be a year apart.
Look at the leaf. Fresh red tea has some sheen, and old red tea is uniformly dull.
Smell it. A fresh tea smells sweet and malty, and an old one smells of very little.
A dusty smell is the classic sign of tea that has sat too long in a warehouse.
Look at the liquor. Fresh red tea is bright, and old red tea is flatter in colour as well as in taste.
Taste it black. The muscatel note in a Darjeeling is the first thing to go, usually within two years.
Brew it and wait. A fresh tea leaves an aftertaste, and an old one tends not to.
Compare it against a blend you buy fresh, which is a surprisingly good reference for what flat tastes like.
Check the best-before date on anything packed for retail, and treat it as a guide rather than a law.
If a seller cannot tell you the harvest year, assume the worst and price accordingly.
A Darjeeling picked in June is at its best until the following spring, which is nine months at the outside.
Storage caveat
Storage for red tea is about slowing the fade rather than encouraging anything.
Keep it sealed, dry and dark, in a tin or a bag rather than in the paper it arrived in.
A cupboard is fine. Red tea does not need the fridge, and the fridge brings moisture with it.
Keep it away from spices and coffee, because it picks up smells readily.
Buy in quantities you will use within about six months, which for most people means 250 grams at a time.
A bag opened every day for two months is exposed every day, so decant a week’s worth instead.
Do not decant into a decorative tin that does not seal, which is the commonest storage mistake.
Write the purchase date on the bag, because after that it becomes a guess.
If it smells of nothing when you open the bag, it will taste of nothing in the cup.
There is no way of keeping red tea that makes it better, only ways that slow it down.
At 4 grams a cup, 250 grams is about 60 cups, so a bag opened daily lasts around two months.