Your metabolism is controlled by your diet and your activity level not your age : ) My metabolism is faster now at 40 than it ever was in my late 20's and early 30's because I workout and eat completely different now than I did then. I used to struggle to maintain my weight now I have zero issues with weight maintenance.
My diet is cleaner 80% of the time (80% of the time I eat organic, no processed or fast food, and tons of veggies, fruit, lean proteins, and healthy fats, 20% of the time I eat whatever I want).
I also do
Metabolic Conditioning workouts and I do everything I can to boost my immune system by getting enough sleep, taking vitamins and immune system supplements plus I make an effort to reduce stress which can also slow down metabolic function.
Having a slow metabolism is a choice so choose to not allow that to happen and it won't