Processing Speed by Age

Here is the method, stated plainly so you can argue with it.

Studies that time adults of different ages report the same shape over and over. Speed measures fall by roughly two hundredths of a standard deviation per year of adult life. Spread that across the width of a typical speed distribution and you land near three quarters of one per cent a year.

That is the whole model. Start at 100 for the youngest band, then take off about that much for every year of a band's midpoint. The middle column is the result.

The raw numbers are deliberately missing. A digit symbol count depends on how long the clock runs and how the symbols are drawn. A decision time depends on your monitor and your mouse. Percentages survive all of that, so a percentage is what this page can honestly publish.

Our own published sample size is zero. Nothing here is measured from visitors to this site.

Read the third column before you read the second. It holds the middle half of each band, the range that 50 people out of every 100 fall inside.

The bands overlap heavily. The middle half of the 65 to 74 band runs from 60 to 82. The middle half of the 35 to 44 band runs from 78 to 106. Those two ranges share a stretch, so a quick person in their late sixties outpaces a slow person in their late thirties.

This is the line most age charts leave out, and it is why no single score can place anyone's age. Age accounts for a small slice of why two people differ on a timed task. Everything else about them accounts for the rest.

Very little of it is your muscles or your nerves. Signals do travel slightly slower along a nerve as you age, and the effect is tiny. Timed scores change far more than that can account for.

Most of the change is in the wiring between distant parts of the brain. Those long connections are wrapped in a fatty sheath that works like the insulation on a cable. The wrapping thins with age, so a signal crossing the brain arrives a little late and a little smeared.

Some of it is not the brain at all. It is choice. Older adults tend to trade speed for accuracy and to check before they commit. When a study measures that trade separately, a real share of the age gap turns out to be caution rather than capacity.

The rest is the equipment in front of you, and none of that has anything to do with age.

The age curve for speed is not the age curve for a mind.

What you know moves the other way. Vocabulary, general knowledge and the skills of a job you have done for years keep climbing well into the sixties. A task that rewards knowing things shows the opposite slope to the one on this page.

That is why a speed measure on its own is a poor summary of anybody. It is narrow and cleanly measured, and the narrowness is exactly what makes it worth timing.

• Timed tasks with no knowledge in them get slower, gradually, from the mid twenties.
• Tasks that reward what you have learned keep improving for decades after that.
• The two are measured separately, and this page measures only the first.

It cannot work out your age from your score. The bands overlap far too much for that, and any site claiming otherwise is selling something.

It cannot say whether your own speed is normal for you. It has no idea what your monitor, your mouse or your trackpad add. The same person on two machines can land tens of milliseconds apart.

It cannot detect anything. A slow run is a slow run, on one afternoon, on one machine. Tiredness, caffeine, an interruption and the tab playing music behind you all move a speed score. Worries about a change in yourself, or in someone close to you, belong with a doctor rather than a web page.

Compare yourself against your own earlier runs on the same machine first. That comparison takes the hardware out, and the hardware is the biggest thing this table cannot see.

Then treat the band as a rough sanity check and nothing more. The digit symbol substitution test gives a count over a fixed ninety seconds. The symbol search test gives a median decision time. The number comparison test gives a rate in pairs a minute. They are three views of the same underlying speed.

The mental age test is the one page here that runs the curve backwards. It takes two timed blocks and names the band they land nearest. Read it as an illustration of the overlap above rather than as a result about you.

Average reaction time by age shows a related curve for a much simpler task, and the slope there is gentler. What counts as a good reaction time makes the wider case for why one number is a weak summary of a person.

The slope above is simple arithmetic on a published effect size. It is easy to check and easy to replace, and it gets replaced by our own figures the moment a cell holds a thousand results.