How come when we age, we tend to feel more tired?

When I was in my 20s, I had more energy than I do now, even when I didn’t get much sleep back then. Now that I’m in my early 30s, I notice that it takes more energy to do certain things, stay awake, lift weights, take care of my kids, and so forth. What are the physical processes that make us more tired and have less stamina as we age? Just curious, because I do a lot of swimming and weight training exercises.