How processing speed changes across adulthood
Processing speed tracks age more closely than anything else a cognitive test measures. It is also the last thing you would notice about yourself, because ordinary life covers for you.
What processing speed is
Processing speed is how fast you do one simple mental operation and get it right. The operations stay trivial on purpose: match a symbol to a number, pick the shape that differs, say whether two strings are identical. You will get them right, every one. The clock is what the test is watching.
The easiness is the point. It keeps your score clear of what you studied and what you happen to know, which is why researchers keep reaching for the measure. It is also why the test feels like an insult while you sit it. The items look like nothing.
The general pattern
In the large studies, average scores on speeded tasks peak somewhere between the late teens and the mid twenties, then slide. Year to year, you would never see it. Stretch the window out to decades and it shows up in study after study.
That is an average across a lot of people, and it matters which way round you read it. At any age the spread between individuals runs much wider than the gap between age groups. A fast 60-year-old beats a slow 25-year-old without breaking a sweat, and researchers do not find that odd.
I feel this one myself. My raw reaction time at 48 is worse than it was at 25, and yet on a circuit I know well I am faster than I used to be. Both true. I have stopped reacting to most of what happens, because I know it is coming. Anticipation took over the job my reflexes used to do. Most people build some version of that, which is why the decline reads clean off a test and almost never off a life.
What does not decline
You miss the change in your own processing speed because other things are moving the opposite way. Vocabulary, general knowledge and whatever expertise your work has built in you keep piling up into later life. Experience sharpens your sense of which problems deserve any effort at all.
Someone slower at symbol matching at 55 than at 25 can be far better at the job itself, because the job pays for knowing what matters, not for snapping at arbitrary symbols. Psychologists split that into fluid and crystallised abilities. One drifts down, the other climbs.
What moves it in the short term
Before you read anything into a slow result, rule out the dull explanations first. Your state on the day moves processing speed more than you would guess:
- Sleep. A short night costs you reaction time long before it touches your accuracy. You still get the answer right, only later.
- Input device. A thumb on glass is slower than a hand on a mouse, and both lose to a key you can rest a finger on.
- Divided attention. Conversation in the room and notifications on the screen cost you more on a speeded task than on an accuracy one.
- Caffeine and time of day. Both move your reaction time by a measurable amount, and which direction depends on you.
Any one of those can open a gap wider than a decade of age. That alone is reason enough to hold a single online test result loosely.
Seeing it in your own numbers
For an honest read on your own speed, fix the conditions rather than take more tests. Same device, same sort of hour, notifications off. Then compare across weeks instead of across several goes in one afternoon.
The absolute number leans hard on which test you took, so by itself it says little. Steadiness says more. A number that lurches around under steady conditions is usually reporting on your attention that day rather than on your speed.
The honest summary
Processing speed falls with age, on average, from early adulthood, and it falls slowly. Differences between individuals dwarf the age effect. Knowledge and judgement run the other way, which is why the decline stays invisible in an ordinary life. And one short online test, taken on a phone, with a notification arriving halfway through, is not evidence of much at all.