Biological age: how old your body really is.

Biological age is the age your body shows on the inside: how well your heart, muscles, metabolism and cells are working. Chronological age counts the years gone by; biological age, how you've lived them. The two can be years apart in either direction, and the part that's down to you is the biggest.

Your passport says you're fifty-two. At dinner you meet someone your age who looks forty-five, and another who could pass for sixty. I see it often in the people I work with: a date of birth says little.

Biological age vs chronological age: what's the difference?

You can't change your chronological age; your biological one, you can. Genes are thought to decide between a fifth and a third, and food, movement, sleep, stress and surroundings do the rest.

• Your body doesn't age all at once. Of over five thousand adults, almost one in five had an organ that had aged early.
• Four habits are worth years. In over twenty thousand people followed for about eleven years, those who didn't smoke, stayed active, drank little and ate at least five portions of fruit and veg a day had the risk of death of someone fourteen years younger.
• It starts in your cells. With age, their power stations produce less and low-level inflammation sets in. Exercise works on the first, sleep on the second.

How is biological age calculated?

It's estimated, because there's no exact figure. Epigenetic clocks, the best-known tests, read tiny chemical tags on the DNA in a blood sample and work out an age.

DNA tests. They work well in research, but for an individual the result can vary between samples, sometimes just because of the lab. The most useful tells you how fast you're ageing.

Standard blood tests. Blood sugar, inflammation and blood fats, checked once or twice a year and read by your doctor, are the most practical clock.

How can you tell your biological age without a test?

Look at fitness and strength. Aerobic capacity, or VO2 max, is the sign most closely linked to lifespan. In large studies, a weak grip is a stronger warning sign than high blood pressure.

Your fitness. Pick a regular hill or three flights of stairs and note how you feel at the top, at a normal pace. Repeat it monthly: if your breath holds up better, your engine's improving.

Your strength. Grip takes a minute to measure with a dynamometer: ask your doctor or trainer. With age the body sways more; the stronger you are, the steadier you stay.

The habits that lower your biological age

Strength, twice a week. Half an hour each time, with weights, bands or body weight: that's most of the benefit. Muscle soaks up much of the sugar from a meal and keeps you steady on your feet.

Fitness at two speeds. Plenty of easy cardio plus one hard interval session a week, if your doctor agrees.

Seven or eight hours' sleep. At night your body lowers inflammation and repairs itself.

Less added sugar. In a study of a few hundred women, more added sugar sped up the DNA clock even in those who ate well.

People. Strong relationships matter as much for how long you live as giving up smoking. Book dinner with friends in your diary, like a meeting.

If you pick just one thing, start with strength: two sessions in your diary this week.