- As we age, losing weight can become more challenging, and one of the key reasons is a natural decline in metabolic rate. Did you know that after age 30, your metabolism starts to slow down by about 1% per decade? That means, by the time you hit your 40s or 50s, your body is burning significantly fewer calories at rest than it did when you were younger. The slower your metabolism, the more difficult it becomes to lose weight, even if your diet and exercise routine stay the same. FitnessThreads