Chronological Age vs. Biological Metabolic Age
Chronological age is the count of calendar years you have lived. "Metabolic age" has no clinical definition. Body-composition scales use the term for a comparison against typical values for your age, and this calculator does the same from your body fat and muscle mass percentages. It does not measure your basal metabolic rate (BMR); it uses its own simple model, set out in the key facts, to turn those two percentages into a number of years.
Your BMR is highly influenced by body composition. Muscle tissue is metabolically active and burns calories even at rest, whereas fat tissue has low metabolic activity. A person with high muscle mass and low body fat therefore tends to have a higher BMR for their weight. This calculator does not estimate BMR itself; it only reads that body composition, so a younger result says your fat and muscle percentages compare well with typical values, not that your metabolism was measured as faster.