Where does the idea of mental age come from?

It started with a practical problem. "In October, 1904," Alfred Binet and Théodore Simon wrote, "the Minister of Public Instruction named a commission". It was looking at which children French schools should teach in special classes.

The commission ruled that no child should be moved to such a class "without first being subjected to a pedagogical and medical examination". It didn't say how that examination should work. Binet and Simon set out to supply one, and they were clear about why it had to be a measurement: "Quantitative differences, such as we have noted, are of no value unless they are measured."

Their first scale, in 1905, "consisted merely of 30 tests, arranged roughly in order of difficulty", as the American psychologist Lewis Terman later described it. By 1908 they had arranged the tasks by age, trying them on about 200 children aged 3 to 15 to see what children of each age could usually do.

Grading tasks by age made a new kind of measurement possible. Terman, who adapted the scale for American schools, explained it in 1916: if a 9-year-old "goes only as far as normal 8-year-old children ordinarily go, we say that his 'mental age' is 8 years."

Set mental age beside real age and you get a single number. "The intelligence quotient (often designated as I Q)", Terman wrote, "is the ratio of mental age to chronological age." A 10-year-old with a mental age of 10 scored 100. A mental age of 12 at that age scored 120, and a mental age of 8 scored 80.

The scale was tuned so that the middle child of each age scored 100. A correct scale, Terman wrote, "must give a median intelligence quotient of unity, or 100 per cent, for unselected children of each age". In his classification, 90 to 110 counted as "normal, or average, intelligence".

The word "normal" in these old texts reflects how children were described at the time. The scales were built to compare a child with what was typical for their age, nothing more.

The ratio only works while abilities keep growing year by year, and in adults they stop. David Wechsler, who designed an intelligence scale for adults, pointed out that "beyond the age of 15, mean scores on practically all intelligence scales cease to increase significantly with age".

He gave a simple example of why: "A mental age score of 12 years, for example, in a child of twelve does not represent the same level of mental ability that it would in the case of a man of forty."

That makes an adult mental age meaningless. "To speak of an individual having an M.A. of 20 years", he wrote, "is both practically and scientifically meaningless." Terman had already worked around the problem by setting aside any age above 16 when working out an IQ.

Testers had tried to patch this by dividing every adult's mental age by the same assumed age. Wechsler wrote that adult IQs "have been almost universally calculated by using some assumed constant denominator", which meant psychologists were "calculating not intelligence quotients but efficiency quotients".

Wechsler's fix was to compare adults with people of their own age instead, and that is how professional intelligence scores work today. Mental age, as a measure of ability, quietly left serious testing.

Psychologists still study a different kind of age: subjective age, the age people think of themselves as being. In a 2006 study, David Rubin and Dorthe Berntsen interviewed a representative sample of 1,470 Danish adults aged 20 to 97 in their own homes.

• Adults younger than 25 tended to feel older than they were.
• Adults older than 25 tended to feel younger than they were.
• From age 40 on, the gap held steady as a share of age: older respondents felt about 20% younger than their actual age.
• Gender, income and education explained very little of the difference.

The authors read this pattern as support for a lifespan view of subjective age, in which how old people feel shifts with their stage of life, over the idea that people simply deny getting older.

That is much closer to the spirit of our quiz than Binet's tests ever were. It isn't about ability at all. It's about how old your everyday life feels.

• 15 light questions about everyday habits and taste: late Friday nights, unknown callers, canceled plans, open browser tabs, unasked-for advice, early nights, weekend plans, group chats, new slang and what counts as a treat.
• Each answer leans younger or older. The quiz averages them and turns the result into an age between 14 and 70, so a typical mix of answers lands in the early thirties.
• Five bands: The Live Wire (up to 19), The Explorer (20 to 26), The Juggler (27 to 33), The Steady Hand (34 to 45) and The Old Soul (46 and over).
• If you like, you can enter your real age to see the gap. That number stays in your browser and isn't part of the result you share.

Some details of the scoring, for the curious. Choosing the youngest-leaning answer to every question gives 14, and the oldest-leaning gives 70. The answers are not listed in age order, so always picking the first option gives 28. And because the average of 15 answers barely moves, the quiz stretches results outward from the middle; without that, nearly everyone would land between 38 and 46.

The weights behind each answer were written to be playful and recognizable, not taken from a survey of what people of each age actually do. So read your result as a reflection of your habits, not a fact about your mind.

This guide has met three different ages. Binet's mental age measured what a child could do. Rubin and Berntsen's subjective age is how old a person feels. Our quiz measures neither: it reads your habits and gives them a playful age, which is why it can say 14 or 70 without claiming anything about your ability.

• Answer for how you actually live now, not how you'd like to live.
• Treat it as a conversation starter, not a verdict. The fun is in comparing answers with friends.
• If your result is far from your real age, look at which answers pulled it there. That is the interesting part.
• Your habits change with a new job, a new city or a new routine, and your result could change with them.
• Take it again in a year and see whether your evenings have aged.

This quiz is for fun. It measures habits and taste, not intelligence, maturity or anything about your health.