Does your metabolism slow with age and make you gain weight?

Short answer

No. Once body size is taken into account, the calories you burn each day stayed the same up to age 60, then fell by 0.7% a year from about age 63. Weight gained in middle age rose along with changes in food and movement.

Does your metabolism slow with age and make you gain weight?
'Older → slower metabolism → weight gain' — does this formula hold?
A common belief is that metabolism drops as you age, so eating the same amount makes you gain weight. We checked when metabolism really starts to drop, and where middle-age weight gain comes from.
When does metabolism start to drop?
It held steady until age 60
A study that precisely measured daily calorie burn in 6,421 people from 29 countries found that, after accounting for lean mass (muscle and everything except fat) and body fat, daily calorie burn was constant from age 20 to 60. Lean mass did not change over that span either.
The drop comes after 60
7% a year from about age 63
In the same study, daily calorie burn fell by 0.7% a year from about age 63, and the oldest people burned about 26% less than middle-aged adults. On 2,000 kcal a day, that is about 14 kcal less in 1 year. Even during this decline, body fat did not rise.
So what causes the drop?
Less of the muscle and organs that burn the most calories
A study comparing resting calorie burn in 26 young people and 26 people aged 60 to 82 found the older group burned less because they had less of the high-burning tissue, such as muscle and liver. There was no finding that each tissue itself burned fewer calories.
Then where does middle-age weight gain come from?
It rose as eating and movement changed
Tracking 120,877 people for up to 20 years, they gained about 1.5 kg every 4 years on average. Eating potato chips one more time a day meant 0.77 kg more over 4 years; 1 more hour of TV a day meant 0.14 kg more. Those who increased activity the most gained 0.8 kg less than those who cut it the most.
Keep your muscle, keep your calorie burn
12 weeks of strength training raised calorie needs by about 15%
When 12 people aged 56 to 80 did 12 weeks of strength training, body fat fell 1.8 kg, lean mass rose 1.4 kg, and the calories needed to hold their weight rose about 15%. An analysis that pooled 18 studies also found strength training raised resting calorie burn by 96 kcal a day.
Summary: before 60, metabolism is not to blame
The formula breaks at the 'older → slower metabolism' link
Daily calorie burn stayed the same until 60, and even after that it fell slowly, by 0.7% a year. Middle-age weight gain rose along with changes in food and movement. Keeping muscle through strength training also kept calorie burn up.