What Causes Slow Heart Rate?

I working out every day for the past 3 1/2 months and my resting heart rate now is 50 to 55. I am 55 years old. Isn t is unusual for my resting heart rate to decline. I have lost about 15 lbs. I have had my heart rate during exercising go as high as 168. I feel good but I am concerned about the slow rate.