Why does a year feel so much shorter at forty than at ten - is there an actual explanation or is it folklore?

On the proportional explanation you have half-rejected: it is worth knowing it is an old idea, that it does capture something, and that your objection to it is a good one.

The idea is that each interval is judged against the length of life so far, so a year at fifty is a fiftieth and at ten it is a tenth. It produces a curve that matches the general shape of what people report.

What it does not explain:

The lurches. It predicts smooth compression, and people report specific years as long or short.

Why a holiday feels long afterwards and short while it is happening, which is a well-known and slightly paradoxical pattern.

Why the effect can reverse. Retire, move country, or change everything, and people frequently report time slowing again - which a pure function of age cannot produce.

The encoding account handles all three, which is why it has largely displaced the proportional one as the main explanation. They are not incompatible; the proportional element may contribute.

The holiday paradox is the clearest demonstration that these are two different clocks: in the moment, a novel week is busy and passes quickly; looked back on, it is full of distinct memories and reads as long. Ordinary weeks do the opposite.

You have put your finger on the important distinction, and it is the key to the whole thing: this is about memory, not about experience.

Ask somebody how fast this morning is going and age makes little difference. Ask how fast the last decade went and it makes an enormous one. So whatever is happening, it is happening in retrospect.

The leading account is about how densely a period is encoded. Your brain does not store a continuous recording; it stores distinguishable events. When you look back, the length of a period is judged largely by how much retrievable material is in it.

A year at ten contains an enormous amount of first-time experience - new school, new skills, new places, everything unfamiliar and therefore encoded in detail. A year at forty, with a stable job, a familiar route and a routine, contains far fewer distinguishable events. Same duration, far less to retrieve, so it reads as shorter.

That also explains the lurches you noticed, which the ratio theory cannot. A year with a move, a new job or a child in it feels long in retrospect, at any age. A year in which nothing changed compresses to almost nothing, and that is why the effect is not smooth.

The practical consequence, which is the part I find genuinely useful: if the effect comes from a lack of distinguishable events, it is partly under your control.

Things that reliably make a period feel longer in retrospect:

Novelty, in any amount. A new route to work, an unfamiliar place at the weekend, a skill you are bad at. It does not have to be dramatic; it has to be different enough to be encoded separately.

Travel, which is the strongest version and why a two week trip can outweigh the three months around it in memory.

Marking things. Photographs, a few lines written down, anything that creates a retrievable anchor. People who keep even a sparse journal consistently report a fuller sense of their own past.

Learning something, because early learning is dense in a way that competence is not.

What compresses time: identical weeks, the same route, the same rooms, and: increasingly: days where the memorable content was on a screen and is indistinguishable from the day before.

None of this changes how fast Tuesday goes. It changes how much of this year exists when you look back at it in December, which is the thing you are actually noticing.